Epic lag problem, need help?

I have a problem with my PC. When I plat a game, any game, it lags for about 5 minutes, then it's normal for about 1 minute, then lag again and so on and son on. I can run all the games I've tried without a problem on my PC, I meet the recomanded requierments but still ..... I'm using XP if it makes any differance, thanks