My only suggestion is, when it's acting up do the following if you're in windows:
run ---) CMD ---) once in CMD, type the following:
ping www.google.com (any website will work, preferably you'd try a few websites that you know the exact location of their servers - for example Whitehouse.gov is out of Chicago, Google is in California, similar type things for EU land) anyhow:
ping www.google.com -n 50
then it will run the test for a while, if you're having packet loss the report at the end will inform you of that. If you can get a test with packet loss, you can also do a tracert on the offending website and do a little sleuthing to figure out what server is killing you by looking up where the IP is from geographically.
you would type into CMD: tracert www.google.com
Once you find an IP that's repeatedly killing you + some packet loss, you can call your ISP and get an explanation, perhaps a fix, or at least a timetable for it to be fixed. If you look around google you may also be able to find a website or forum where things relating to your network are discussed which can serve as a go to when you're having trouble. I know my ISP's forums can be found on google and generally any time my internet is messed up I can go there and see various people discussing the problem specifically so it saves me the trouble of calling and complaining lol.
But it could be anything from a DNS server problem, local line issues (fiber cables messed up or phone lines messed up if you're on DSL), or some server hub getting too full/having problems.