Hardware issue

Hello,

I start up my system and it runs for awhile, then the monitor light goes yellow and loses video.
I figured it was the mem but i remove the mem and get bbeps 1-3-2 mem not seen.
i put inthe mem it boots fine for awhile . It seems like it goes into sleep mode when your away and will not come back.
I have a dell gx280 sff(small form factor) desktop. I have no diagnostic partition to test the hardware.
The bios recognizes the mem and the processor heat is fine but the cpu fan races every once in awhile.
I have to unplug the power cord from the machine to reboot whenever the light goes yellow or the fan revs. but after that it shuts down and restarts no problem.As soon as it goes to sleep it seems the machine needs to be unplugged to restart.Any help would be greatly appreciated
Dave

Ps.- i thought it might be a heat issue but the machine does not get hot. Iwas wondering if it might be the power supply or maybe the sensor on the fan?