Weird issue with Assassin's Creed Brotherhood

Hi, recently I've been playing Assassin's Creed Brotherhood, but I have a weird issue with it. The game plays fine for usually around 10-30 minutes, (though it can go as high as 50 occasionally) but then at a random point it just starts lagging like hell, forcing me to restart my PC to get it playable again. At first, I get decent fps with no bugs or issues at all, but then suddenly the game starts lagging as if it's in slow motion when I move around the world; though I can stand still and move the camera around fine. I had similar issues with Assassin's Creed 2 and Batman Arkham City, but in those cases it was much less extreme and usually took at least a couple of hours before the issue set in.

Here are my system specs:
2gb RAM
HD 4650
Petinum 4 3.6ghz

I know my system is terrible, I have very little money so this was the best I could get. I'm assuming it's just an issue with my processor being way below the requirements (Core 2 Duo 1.8ghz), but it works fine with almost all games that require superior processors so I think this issue could possibly be related to something else. Could anyone help me out here, please?

maybe its you tempreture well you play game that puts so much stress on your cpu like AC:B it could heat up causing lag. but assasins creed 2 might not put so much stress. you should try updating the game or getting the latest drivers.