Possibly faulty mobo?

Guys/girls I need your opinion.
My girlfriend has a homebuilt PC, warranty expired, and as of yesterday it started to behave strangely.
I suspect it is a mobo, because I tested it with another PSU in case if it is PSU fault, and other things I could not test, because I do not have extra components.

So here are the symptoms:

Sometimes it won't powerup at all (that does a lot). Sometimes it powers but gets stuck after mem test (once). Sometimes loads normally, but hangs in XP, and everything freezes, even the 4sec power button won't work, it has to be pulled out of electricity (also does a lot).

The last thing I did is that I replaced PSU with new Chieftec 550W, unplugged DVD/RW and HDD from power and data, but I got the same results.

So is this a mobo fault?

From what you're saying, it could be the RAM as well. But sounds like the motherboard is about to bit the dust.

Well, I know that it is not HDD's fault, becuse I unplugged it, and DVD drive also. Both from PSU and SATA/ATA. I couldn't test ram, 'cause the last thing it did, is that it wouldn't power up - again.
Even if ram is faulty, I would get beeping errors, but nothing.
When XP loads and after a time it stucks, with faulty ram, I think it would still be able to shutdown on 4sec PWR button. But that also ain't working.