Dell XPS 8500 wont boot: Mobo?

[a_gunslinger]

Have a newer (2012) XPS 8500 (out of warranty of course).

SYMPTOMS:
Turn on power, CPU fan spins, exhaust fan spins. Then both stop after 2-3 seconds.
After a few seconds they both spin again, but then stop again.
No POST beeps
Yellow power light on Mobo on and yellow.
PSU back light green.

WHAT IVE DONE:
Tested PSU with a plug in tester. All Blue so believe PSU good.
But also swapped PSU with a similar (Dell XPS 8300) - same symptoms
Replaced RAM and did one stick at a time. No love.

Any thoughts/advice/next steps?

All help and advice greatly appreciated!

 

[C12Friedman]

I'm thinking the CMOS battery may be in need of replacement - check that the battery is at 3v or replace, the advantage to doing that is just the act of taking the battery out for more than 10 seconds will reset your BIOS which may be all that's needed. I have seen where Dell machines do not boot if that battery is bad