What mouse do you use?

m_rawinput 1

deathadder

Also, I have a zowie am and I was thinking about just switching over to that, using a different config and just using a different sens but that's probably an awful idea

just keep your current mouse. so one of the updates must have messed up something to do with your config. when you say it happens again do you mean your sens in tf2 changes? It's prob something to do with the config try taking out your config, go to an empty server and try it then if it works put your config back in. (if that breaks it again then something's wrong with your config that conflicts with something)

Is your fps consistent?

Clean your mousepad off and spray the sensor lightly with compressed air.

My mouse (R.A.T.7) does the same thing occasionally and it usually goes away for a bit after I clean it.

yeah but he says it happened to him after an update so i don't think that's it.

It happened after that one update when people started complaining about fps problems, but my fps hasn't been considerably worse or anything

btw did you update your mouse drivers? if it gets bad enough you can always reinstall tf2

Did that and doing that

could it be your mouse pad? just because your problems coincide with a tf2 update mean it was because of the update. coincidences happen.
anyway, i find that my mouse pad gets sections where my mouse moves slower until i give it a real good wash. make sure your sensor is real clean too (my DA had really bad sensor problems where i would suddenly find myself staring at the ground or sky while playing). check for driver updates and all that jazz etc.

if all else fails switch to a different mouse at least for a bit to see if its the mouse or tf2, if its the mouse then there you go, if its tf2 then idk whats wrong.

I just re installed tf2 and tried 3 different mice and its doing the same thing :|

My g400 sometimes switches either profiles or sens randomly ingame for a couple seconds. Not very often, maybe once or twice in a pug, but enough to be annoying. I usually just alt-tab in and out and it works, but yeah, it's annoying

If the in-game sensitivity is changing itself for some reason, you could try this in a config:
bind mouse1 "+attack;sensitivity #"

same thing has been happening to me. I use a g400, and what it feels like to me is that mouse accell is turning itself on and off and its just impossible to stay consistant.

mental

Definitely not, also darkstar is your games Filter setting in advanced video options on trilinear? normally mine is on bilinear and I switched it and it feels a bit better, try it and tell me if it works at all, ty

tried legacy driver instead of synapse2?

alternative is that u folow this guide if u have acceleration enabled from ur windows

http://www.esreality.com/?a=post&id=1846538

is accel on?

check if somehow enhanced pointer precision got turned on in windows??

FYI: Windows sens(including windows accel) is no longer a factor if you can verify that raw input is in fact enabled.

did you keep the same config you had when you reinstalled tf2? i think it might be your config binded something to change your sens and you accidentally hit it otherwise it might be a problem with windows sens.

A decent ammount of TF2 settings are stored in the registry so they would most likely persist assuming you reinstalled TF2 using just Steam. Also did you verify that your whole tf2 folder was deleted along with the uninstallation? iirc the [account name]\team fortress 2\ folder isn't deleted along with the GCF files in the uninstallation, so any scripts or what not which could be affecting your sensitivity will persist through multiple installations.

If none of that was the issue then try this:

1. Open up TF2, go to advanced options, and disable the "Sync with Steam Cloud", or whatever it's called, then close the game.
2. Right click TF2 in your Steam library > click properties > go to the updates tab > uncheck "Enable Steam Cloud synchronization for Team Fortress 2" if it's checked > click close
3. Backup your TF2 launch options in a text file if you'd like, then remove them and just put "-autoconfig - console" and hit OK.
4. Open up regedit and go to HKEY_CURRENT_USER\Software\Valve\Source\tf\ and delete the settings key. If you're unsure it's the correct one it will have many DWORDs starting with the name "mat_" and a few starting with "r_"
5. Remember "Team Fortress 2" folder previously mentioned? Back that up by moving it to your desktop for example.
6. Launch TF2, bring up the console, and type "exec config_default", either that or default_config, I forget. Now close your game and remove -autoconfig from the launch options so that just -console remains.
7. Load up TF2 again, bring up console, and type "map itemtest", when it loads change your "fov_desired" to what you usually play at, as well as "sensitivity" and any other custom mouse movement related CVARs you might use. Then just play for a bit and see how your aim is as you're playing on the closest thing to default TF2. If the issue still arises try playing with m_rawinput 1 (if you don't already), and if that doesn't fix it then it might be a Windows problem in which I would have to research further.

EDIT: Also make sure you're not using an exorbitantly high anti aliasing settings (more than 4x) or anisotropic filtering settings (more than 8x) as they introduce input lag at higher settings since iirc they are done after the main image is rendered and the higher the setting the more AA/AF passes need to be done.

are you able to measure this fluctuation? measure in/360 before and after. See if this new sensitivity is consistent. Does it depend on the original sensitivity?

put sensitivity in autoexec and make it read only :)

Good point, should always rule out a possible placebo in these cases.

I have a similar problem but I'm pretty sure it's cause I use pointer precision. Either way I feel your pain, hope you find a fix.

I had the same problem before. I opened up Razer Synapse or what every it's called and that seemed to fix it.

Have you tried different USB ports? It could be a USB port failing on your motherboard.