I have an older 360 that just came into my possession a week ago. I bought the NYKO Intercooler that snaps onto the back of the 360 and helps keep it cool, it seems to be working well...the air coming out the back is certainly warm but I wouldn't say hot. Anyway, I've been playing a few backups...all burned on Verbatim @ 2.4x. Forza Motorsport 3 froze three times while playing last night, meanwhile I haven't had any other game freeze yet.
However, I don't think I've played any other game longer than I have Forza in one sitting. I'm not sure if it was the image I burned that might be bad, the individual burn, or something with my console. I obviously hope it's not the latter, but I don't think it is. A friend and I played Borderlands for a number of hours yesterday without issue.
Also if it means anything, the lockups were a little different each time. One time the screen just turned gray with the music playing, we were able to hit the Xbox button on the controller and go back to the dashboard. Another time it froze in the middle of a race and even the Xbox button didn't work, the last time it froze immediately after a race. Perhaps I'll reburn the game, but I use the same type of disc and same method as I have other games so I'm not sure what could be wrong with it.
I would get rid of the NYKO intercooler before it causes any problems. Try running the 360 without it. If you continue to get freezing issues, you are probably looking at RRoD sometime in the near future.
What is the problem with the intercooler? From what I read, there used to be an issue with them since they drew power directly from the 360. The newer models have their own power supply and had 5/5 eggs on NewEgg from nearly 300 reviewers.