[Jason]

My PS3 freezes after like 20 mins of play.

[Android]

Your PS3 could be having temperature issues.

Unplug your PS3 and leave it for a few days and let it cool down.
Leave it out of the sun and keep it in a well-ventilated area.

If it still doesn't work and keeps freezing all of the time then go to the PlayStation Support page (see link below).

[TylerM]

The best way to try to stop this besides getting a new one is buying an air vent for it, i think Game Stop has them =] hope this helps