thats because the ps3 is old and games are advancing(the slim model and fat ones have no difference), plus in a year and a half the new ps4(hopefully it will have ps3/ps2 compatibility) should release(every 6 years).
Sorry OP, but don't listen to this ^^, it's just nonsense. If that were the case Uncharted 2 and Killzone 2 would make your PS3 blow up...
It doesn't have anything to do with how old the PS3 is it's how efficiently the game engine runs which, as we all know, isn't the best on any platform. Try giving your PS3 a break if you're having a lot of problems. I only play for an hour or two at a time and then turn off my system; I've yet to have any real issues/freezing thus far. Maybe this is related....