Need help with Dual G5

Hi all,
I switched from G4 to G5 some 18 months ago and everything was fine with the new machine until about a month ago.

I'm having problems with starting it up.
When I press the power button I hear the "start-up" sound and after a while all fans (coolers) start to work on a maximum level. At the same time on the screen I see only the grey screen with an apple and nothing is happening.
After 10-15 times trying I'managed to start it up and than it is working fine until I shut it down. And than everything the same...

I sent it to the local service for a repair, but after a month of doing nothing with my computer, they called me and told me that the mainboard is dead or some of the processors, but they cannot test it.
They also said that I should just start it up and never turn it off again and it will be fine!!!???

I would appreciate if someone can help me resolving this situation, since it is unlikely that mainboard is dead and that I can still start it up.

Thanks