GA-7VAX boot problem. Shuts down after 3-5 secs

Hi,

just as some others I'm having serious problems getting my almost new GA-7VAX (Rev. 1.1) to work with my also new XP2400+ (266). Everytime I boot up the machine it will shut itself down after 3-4 seconds. No sound, no beep, fans spinning, monitor stays off.

I don't think this is a hardware issue, more a compatibility with the GA-7VAX. Here is why !
The Mobo has been working for the last two weeks (and still works) with my trusty old Tbird 1400, therefore I'm sure the mobo is not the cause. I just spend my whole weekend getting things to work with the XP2400. To no avail. I harvested through all related posts on several forums, followed all advise I could find but nothing helped in the end.

Well here's what I've done so far. Maybey somebody comes up with further ideas.

First off all on Saturday I went off to buy a new PSU replacing my old 300W one with a 420W by Levicom. No change.
Then I doublechecked the heat sink don't know how many times, refitted processor don't ask how many times, resetted cmos, changed all kinds of stuff in bios (like voltage setting, turned off auto sitch-off for overheating etc. using Tbird 1400), used FSB 100/133/166, changed multiplier manually, used multiplier auto setting, took out all additional cards and stuff, booted with motherboard fitted to the case or unscrewed, tried 3 different fans (the processor remains stone cold btw.), updated BIOS to latest version (F6) I think.

Nothing of that changed even a little bit. All the time fitting the old processor things worked alright.
Fine I thought must be a faulty processor. Took the darn thing to work to return it to the provider. In the office just for the sake of it I fitted it into another machine, put it in an QDI-Kinetiz7e. Guess what happened, the machine boots just fine.

Now I'm really puzzled. All parts work individually, but not together. Any thoughts what I should and could do, other than buy a new mobo ?

Well, I almost forgot. My stats are.

Athlon 1400, GA-7VAX, 60GB Western ATA133, 512MB DDR333 (2700), Soudblaster Audigy2, 3COM FastEthernet, RADEON8500 64MB (one of the first batch), Plextor CDRW 10x16x40, AOPEN DVD 10x40,

Any help is appreciated. Thank you in advance