Won't Boot, Out of Ideas

Hey y'all-

Got an old machine I built that booted a year ago, now won't boot up / stuck in boot loop. Out of ideas on what might be the cause & solution. Quick run down:

It powers on, spins up, then the heatsink fan stops, then 4-5 seconds later entire system shuts off. Boot loop ensues.

I removed a few PCI cards, the gfx card, the memory, the HDDs, the CMOS battery, but no dice. Any thoughts on what I should try next? I've had this issue on and off with the computer before but never been unable to solve.

Any thoughts appreciated. Thanks!