The basic problem is the phone number for out-of-warranty service on their website was wrong. So I got to regular warranty service, and the guy takes my info and then says no, and transfers me. Ok, eventually I get to somebody to whom I can pay money for out-of-warranty service, and then the transfer comedy begins. Every single transfer, I have to tell them the order# or case#, and then tell them my name, phone number, and (sometimes) email address so they can verify that I'm the person with that order# or case#. Plus the endless holds without speakerphone. And then they decide they're not the right person and transfer me. I have five phone numbers written down that I was told I was being transferred to. Half the transfers put me back to regular warranty service (why they can't do the diagonistics once I'm paying for it? who knows).
Eventually a transfer failed and I was disconnected. So I called the number that had come up twice as definitely for sure the right number this time, and that inexplicably was answered by a sane, totally comprehensible tech guy who got me into the dell diagonstics stuff and after some discussion with him about the various things I had tried (swapping RAM etc., and my results of trying different drives and such) he's like 'ok, you totally know what you're doing, so, when these diagonistics finish in two hours, do what they tell you, and if they show nothing wrong, it's either the power supply or the motherboard, and who knows. it's probably the motherboard, but the power supply is the cheaper thing to try first'. The diagonistics found nothing (no surprise, the machine only reboots under fairly specific conditions--totally common in regular use, but not in synthetic testing).
Then the spare parts place (which some of the website claims is available online! but it's not!) says these parts (for a 2005 machine) are in low supply and he'll have to send a request to the warehouse and I'll hear back within 24 hours. So much for overnighting the fix or anything. $50 warranty call + $35 power supply + $140 MB + shipping to fix a $950 computer 3 months out of warranty (and which, sadly, I didn't use for the first six months, so if I'd just waited...).
Of course current Dell extended warranties look like ~$100/year, so it roughly balances out financially, averaged over both my dell desktops.