I don't know if this will be of any use to you, as I don't know what set-up you use - but I had this very same problem that persisted for weeks on end and drove me absolutely crazy - it would always happen in game at the worst opportune moment, and I would end up dead when I managed to get back into the game. It happened with ALL of my games, and when it started happening with other software like Word, for example, as well, then I really knew something was up.
Turns out, we had recently set up a wireless network at home (previously had used cabled internet). The software that came with the USB wireless adapter kept "hijacking" the system, at random intervals from once every 30 seconds or so to once every couple of days. What the purpose was of the hijacking, I don't know - but disabling the software solved the problem and now everything is as it should be.
Have you, by any chance, changed something in your network at home in a similar fashion?! This of course may not be your issue at all, but it's worth suggesting anyway, just in case.