PC Games Running Choppy?

i have the following PC configuration for about 1 year:

AMD Athlon x3 450
ATI HD 5670
MB Gigabyte GA-MA78LMT-S2
Sirtec ATX-410-212
Win 7 32 bit Home Premium
Kaspersky antivirus
SiliconPower DDR3 4 GB

I have a rather peculiar problem with my Desktop PC.The problem appeared a couple of months ago, while I was playing Skyrim. It started to lag pretty badly, even on the lowest settings. I would have understood if it was lagging from the beginning, probably my pc couldn't handle the game, but the strange thing is that it was working just fine before, no lag, constant fps (above 30 on medium settings), now it�s like 15-20 fps. I think "choppy" is the perfect word to describe my gaming experience for the last months. I mean it's not that "game-breaking" lag that completely ruins your experience but enough to annoy you.I have this problem with all of my other games too, games that worked perfectly before. I'm pretty pissed off right now, I've tried dusting my pc, scanning for viruses, updating my Windows, changing my ram, updating my drivers, defragging; I've also tryed replacing my psu with a higher power one, but didn't change anything.

Some symptoms that I thought were important (and weird):

-It does not lag in multiplayer, only in single player.

-I�ve tried pulling out the ram stick and putting it back in and for about 5-10 minutes it worked perfectly but then it started to lag again.

-It starts to lag after some time after the install, in the first couple of minutes it works ok but then it lags every time I run it.The temperatures are fine, the graphic card does not pass 60* and my cpu stays at around 40-50* (in game).