Picture Freezing on 360!

I've had my 360 since Christmas and I have never had any problems. I've read alot about the 3 red light problem, and it sounds like the problem I have, except there are no red lights.
I start to play a game, and can play fine for 15 to 20 minutes, then it freezes up and I have to reset the 360. Then this will happen again. I get no red lights flashing on the front of the console, which is why this seems strange.
This only happens if I play a game, if I am on the 360 dashboard, I can leave it all day and the screen wont freeze. I've tried multiple games and different TV's, (including a brand new one that definetly supports PAL60) and I still get this problem. Has anyone heard of this before? and are there ways to fix it?
Any help is much appreciated.