Is this a motherboard problem?

Hi.

Since about 2 weeks ago, my PC started shutting down automatically after 2-3 hours of being turned on. There is no Windows is shutting down sign, the monitor just turns black then the light on the CPU goes off.

PC Specs:
P5LD2 SE Motherboard
Core 2 Duo E6400
HD Powercolor 4850 PCS+
Gigabyte Superb 550P
Kingston 2 gigabyte ram and 2pcs 512mb
HP w2448hc monitor
WD Green 500 Gb
Windows 7 Beta

I have been using this PC for almost a year, specs unchanged except for the Windows 7 beta which I installed in January 2009.

Over the past week I have tried several things to isolate the problem, but all to no avail. These are the things I have done but the shut down problem still remains:

1. Change PSU
2. Change power cable
3. Change Video Card
4. Change AVR
5. Reformat
6. Monitor temperature (never reaches extreme temp - only up to 50C [I live in a tropical country])
7. updated BIOS

The only other possible problems are Mobo, Processor, RAM, and HD.
The technician said it is most likely that the MoBo is faulty.

What are your opinions? It would be greatly appreciated as I am at wit's end.
Thanks in advance.