Mac Pro 2006 Random Power Shuts off

Hi,

I have a similar problem.

My problem is Mac Pro 2006 Random Power Shuts off

Hopefully someone can help me about very strange behavior of my 2006 Mac Pro 1,1 A1186

Random power shuts off, while idling or browsing, The temp monitors on my menu bar is fairly stable at the shut down. I can boot up again right after shut down but I only get black screen instead of gray apple logo screen.

This is what I’ve done so far;

1, I replaced the power supply unit. It didn't fix the problem.

2, I swapped the video card and the memory riser and the modules from another working Mac Pro 1,1.

3, Then I replaced the motherboard and CPU but the problem was still the same.

4, I replaced PRAM battery just to make sure but no change in behavior.

My Mac Pro boots up after reset the SMC (leave the power cord out for a while sometimes overnight) but then shuts off after using for a while.

Now I start thinking that it could be a faulty capacitors.

Any advice please?