It was working fine until it froze once during a session (about 3-4 hours long) of Uncharted. Then, after leaving it off for about 4 hours, I tried to play again and it froze after about 20 minutes =( Help?

Could be bad luck. Clean it out as best you can. May just be building up some dust.

are you having problems with other games also or just uncharted?  because two freezes around the same area in uncharted really dont provide enough evidence towards the ps3 dying

I shall look into cleaning it. I got past the part of the first freeze, but yeah, that could also be it. Still, it better be ok!

Played some games on  a borrowed PS3 slim last week and it froze a couple of times also.  Unless its a constant problem I wouldn't worry to much about it.  Probably will run like new again once you clean it.

maybe the disc is scratched ?? idk 

Your lense is dying.  This happened to me.  Either change it yourself or give Sony 250 dollars.