I had the same thing happen to me, called Dell after the diag lights read in either sequence, 2/3, 3/4, or just 4 light on, this always was happening on reboot. It would freeze in the reboot screen. Well after all that dell finally sent out a Tech to replace my mother board and memory (XPX 600. he did a few things so fast I couldn't do it if they asked.
Well the problem still isn't fixed. On reboot last night the 2/3 lights stated on, I pulled the power plug, restsrted after 30 seconds and the 3/4 lights stayed on. I unplugged it again, pulled out my Z-board USB connector, plugged the system back in, and it was ok. I'll see tonight, just HATE to call Dell and get frustrated over opening the case, and not being able to understand support.