Sudden lag when playing online games!

Hi, so this problem has been going on for a while.. I first noticed it while playing Diablo 3 and now I'm noticing it for Left 4 Dead 2. Basically what happens is that I play the game for a while and it's completely smooth, but ALL OF A SUDDEN there's HUGE continuous lag running at 5 FPS. I noticed that this problem did not happen at all when playing games off-line, so it's not an overheating issue.

However, if I alt-tab out for a few minutes and bring the game back up, it's smooth again - but the cycle repeats.

What I noticed is that the HDD is spiking like CRAZY when this lag issue happens. Here's a picture of my resource monitor: http://i.imgur.com/NiclA.png

Can someone please help me fix this issue. It's driving me insane!