T400 weird boot problem (blank screen)

[Bruce1981]

Hi guys (and girls )
I`ve been very satisfied user of IBM (and later Lenovo user) until now. On my almost 10 year history of using T series (T43->T61->T400) it`s the first time i`ve serious problem.
So, it`s time to get to the point.
My T400 (6475 Intel discrete) won`t boot. When i push the power button, power LED lights on, Capslock and numlock lights blinks (as usual), HDD is working, ODD is working, CPU fan speeds up for the moment and... that`s it. Screen is blank (black, no backlight), nothing is happening. Sometimes BT LED stays on (ussually it`s blinks for a moment after caps/num lock LEDs).
I`ve done CMOS reset ( following the instructions given by FTC in this thread http://forum.thinkpads.com/viewtopic.php?f=2&t=83046) with no result.
But... (there`s always BUT ) when i unplug power cord, remove main battery and disconect CMOS battery, for a few hours (let say overnight), my precious works again as if nothing had happened.
It goes thru every test (from PCDoctor full test, looped for 10, few hours Prime95 , SuperPi so on and so forth.
I`can hibernate computer, put it in sleep mode or turn off and start again - few times, until it freeze again.
What`s going on? I`ve newest BIOS from Lenovo site (3.24), newest AMT firmware, and all the drivers.
I`m worried, that i MIGHT be a north bridge failure, because IMO computer (BIOS) hangs on PCIEx devices initialization...
Did anyone have had similiar problems?
Is there anything i can do besides mainboard replacement?
Thank you in advance for replies.