You have no firewall what so ever? You just connect straight to the internet? That means that even if you have a good virus scanner you could be infected with a number of trojan viruses which would be a number one cause of that kind of problem. If that is the case then google "stinger", download the prog, disconnect from the net and run the prog (it specialises in trojan hunting). Then get yourself a firewall, like kerio or zone alarm (google and you'l find the free versions) and install. Virus scan, and adaware scan - also get spybot as it picks up things that adaware misses (and vice versa).
If you do have a firewall then the above may not apply, but it's still a good idea to do all those things
Are you connecting to the internet through a router? If so you''l need to check it's setting are correct and it doesn't have it's firewall blocking stuff. Same for if you just use a modem.
If it is none of the above then it's probably a physical problem with the line which can cause dropped data packets which would result in your problem, but that's not likely. It could just be your ISP though, but it sounds like something at your end if you got it fixed before. It definitely isn't a browser related issue though if it affects both IE and mozilla.
Apologies if you already know some/a lot of the above but it might help out other people too so I went into a bit more detail, hope you don't mind!