Computer messing up weird problem someone help?

Okay I have this issue its really weird and makin me angry. I've recently upgraded from 512 ram to 1GB so I could play World Of Warcraft ( online game) anyways the problem is I turn on computer. log in play for 1-2 hours then.. Out of the random blue my fps goes from like 20 to 0 ( fps frames per second). This also happens with videos. Like if i watch a movie after 1-2 hours it will be super slow and laggy same thing with other games aswell.. Only way to fix is restart my comp i've let this go for 2-3 weeks but now its pissing me off and im tired of restarting my computer. Any idea whats the problem? My RAM is fine I run on windows XP computer is a HP compaq not super duper old but about 2 years. Any solutions what I could do? thanks sorry for the bad spelling / periods and stuff :x..