I ran the memory test. No problems.
The video card is a Quadro fx540, monitor is Samsung P2070. I do not get a "no signal" message from this monitor. The screen just goes black. Someone once mentioned that I might have too many processes running and I shut down all processes I did not need. Everything was OK for a while, but apparently that was just a coincidence. This is not a new problem; the real annoyance is that it's intermittent --I can go months without any video loss and then suddenly they come in spurts (4 times just today).
If you think the system freezes, and if the system reboots randomly, it could be an indicator of a temperature problem. Pull off the heatsink/fan assembly from the CPU. Clean off the old thermal paste from both the heatsink and the CPU. Reapply Artic Silver 5 thermal paste, and then monitor. I realize that the biggest concern is that the problem is intermittent, but if temps are the problem, you could force your system to heat up by using software called Prime 95. This software will test your CPU's stability by putting maximum load on it. As a result, the CPU and system temps rise considerably.