SCSI resets caused by weird PSU problem

In the followup to last weeks saga of the exploding PSU...

This one really has me stumped:

I've got a lot of equipment in one of my full tower PCs, mainly U160 SCSI
hard disks, and so I figured I needed a fairly beefy PSU.

I bought a 550W Enermax PSU at the same time as upgrading the disks, but
for reasons I couldn't explain, it failed to power up the system. It got
as far as the POST and then I could hear the SCSI disks repeatedly trying
(and failing) to spin up. I gave up with the Enermax and switched back to
an old 235W PSU I had lying around.

Although this configuration worked without problem, I was really nervous
about running so much equipment on just 235W, so eventually I tried yet
another 550W PSU - this time an Antec. Well this worked for about a 3
months, until, well - see the "My PC Exploded" post of 26th June 2004:
news
I switched back to the Enermax, which seemed to work for a few days,
until today. The first symptom was occasional SCSI resets, which became
worse and worse. I had heard that turning off Domain Validation on the
Adaptec 29160N can sometimes prevent this. It did for a couple of hours,
but then things got worse from there. Finally the PC wouldn't even boot,
once again struggling to power up the SCSI disks.

I've now switched back to the puny 235W PSU, and everything is working
fine, but I'm still not keen on using such a small PSU. I should add, that
I've used this 235W PSU in this configuration before, for more than 2
years, without problem (still makes me nervous though).

I can't understand why two different monster 550W PSUs, from two different
manufacturers, cannot cope with the same system that a tiny 235W PSU can
easily cope with, especially such "high quality" brand name products.

I've had all three PSUs from new, and the exploding Antec is still under
warranty (not the other two though).

I must have build around 200 systems in my time, and I've never
encountered a weird problem like this. I've double checked for floating
joints, all cables and connections - including data cables, and verified
the integrity of all the disks at BIOS level. All voltages and electrical
pathways pass OK, but these 550W PSUs just aren't playing the game.

Any clues?

-
[H]omer