I recently had this problem due to a faulty power supply. Apparently if there's enough power to start everything up but the motherboard never receives a "good power signal" (read: data from the PSU) it won't initialize.
Doesn't really seem plausible as he's sometimes able to get into the BIOS, where, if it's able to get into that stable, it has power enough to run.
It really sounds like a bad mobo. If it's under warranty as you said, it's a fairly easy fix if you find 15 minutesish to spare to hook it all up.
Good luck, and if you need any help, PM me.
