Main console Overheating, or PSU overheating.  Place in well vented area, never inside an entertainment center with glass front doors that are closed during operation.  Any reading on the Xb360 would have told you they have overheating problems.  Mine locks up after about 2-3 hours of play, but after rebooting it's fine again for a while. It does not completely lock up, as I can still hear music playing, just locks up during loading. You can take it back, but there is nothing that prevents it fron happening again with the other one.  People just have them placed in bad areas, where air does not flow right or is already by a heat source, no matter WHAT that source is.   You can also buy the aftermarket cooling docks for it.

I really, REALLY hate that they still have not invented a console that DOES NOT slow down when too much is happening on the screen!  That has been the case since the game console was invented.   You would think the idiots would keep the game within the limits of the machine, and not tax it like that.   No one has EVER made a machine that does not slow down when too much is going on.  Here is a hint, do not put more items on the screen than can be handled!  Keep the frame rate constant!  Stop flooding the screen with conent, and that will not happen. That is the constant problem I have with these stupid MFGs!  They never, EVER, have addressed let alone solved that problem.