extremely slow post/boot SB51G

I have an SB51G that is about 3 years old, P4 2.4, 1GB RAM Running XP Pro SP2. When it has fully booted, it is mostly rock solid-- NEVER locks up, no blue screen, no memory errors or windows reporting serious errors, etc. BUT, this may be a clue: very occasionally (once or twice a month) it will reboot with no warning, and I can't find a pattern-- I don't believe it's heat related, I can run Quake for an hour straight and not have an issue, but then another day, my kid could be writing a report and it resets itself.
The big issue is when I first turn it on, every single time it takes about 15 min to POST. It powers on immediately, then hangs just before checking the RAM (with normal video), sits there for the 15 min with no sign of activity, then continues, counts the RAM and boots as normal. Exactly the same thing happens on a normal Windows restart or a hard reset with the button or on the occasional self reset. Very weird. I have tried with no change of behavior:
* Different RAM
* Removing AGP card, HDD, CD, USB card reader, all periferals
* New power supply (currently running an Antec SP-450 externally with the cables running through the hole in the back, further reducing heat)
* Resetting the BIOS via jumper
* Optimized and Fail-safe BIOS settings, lots of tweaking combos
* Latest BIOS firmware
* New CMOS Battery

I don't have another CPU to swap or I would try that,
Once it is going, the only issue is the rare self reset, no other hints. Sometimes it will go for days, I leave it running for 3 or 4 days straight usually.

It is a great machine, I don't want to retire it. I haven't seen this issue anywhere. Any ideas?
thanks,
dan