I just started having an issue where when i start my computer up it starts out going full speed but if i keep it on for awhile then it starts having performance issues like when i play even the smallest game it still lags like crazy i started noticing it when i went to play need for speed underground 2 and that was barely playing but when i restart it goes back to normal and i can play games but i dont like restarting everytime i want to play a game anyone know what the problem is?

oh and just as a benchmark i can normally play games like skyrim at very high settings with no lag of course i have my computer stats on my account

definitely sounds like an overheating issue. Have you tried cleaning out the fans / filters on your computer? also, is the area around the computer clean, no dust that will easily get in? also, can you hear the fan overworking itself constantly, or not working at all?