I downloaded a stress test program called y-cruncher. It can run memory stress routines calculating huge pi numbers. I ran the y-cruncher stress test mode and was able to duplicate the problem. The system crashed and rebooted. This seems to point or confirm that the problem is faulty memory on the M4985-SI ad-on board.
A few day ago I entered the BIOS setup and lowered the memory speed from 667mhz to 533mhz and also disable memory cache buffers. This lowered the system performance somewhat, but made it more stable. I was able to run the y-cruncher stress test and the system held up.
So I will be replacing the faulty memory soon.