A few bucks later, and I think I may have fixed my xbox problems. I played tonight for a few hours with the case fully assembled without it freezing. Though, after a a while it did freeze.

It's definitely a heat problem. I attached Nyko's Intercooler thing in addition to attaching a small fan to the cpu heatsink and covering the ductwork with aluminum foil so that the air being sucked out by the exhaust fans is forced over the cpu and gpu/ram heatsinks.

If it continues to freeze, I'll try another suggested method of removing the heatsinks, cleaning them, and applying new thermal compound.

It's still confusing as to why heat issues are so common on xboxes, and that Microsoft seems ignorant of the problem and is more than happy to charge customers $99 to fix it each time heat causes solder or chipset failure? (In my case I'm strongly leaning towards solder cracks based on the behavior).

At any rate, the additional fans alone weren't enough. Adding the aluminum foil covers to the heatsinks seems to be the magic ticket.