Built system, having boot problems once in a while

Hi, I recently ordered a prebuilt system for my father, nothing fancy, all he needed, and when I first went to boot it up it gave no post beeps and didn't get to the bios screen however all fans and leds did come on. I eventually got it to work but sometimes it would happen again when i shutdown or restarted. I went through tons of memtest and support from NCIX where I bought it. Eventually they agreed that it might be a motherboard problem since we had extensively tested the ram (2x1GB dimms) I sent it back for replacement and got the new one today. I installed it and all was going fine until I was using it and the screen went black, power was still on but computer was non-responsive. I hit the reset button but during boot I got no beeps and it did not make it to the bios, it just sat there with the LED's on and fans running. I took out both ram sticks and put both back in and than it worked fine for a while so I tried restarting it to see if it would do that fine. Unfortuantley I got the same no beeps no post so I took out 1 stick and restarted the computer and have now restarted it fine 3 times with just the one dimm. I'm wondering if this sounds like a memory problem and maybe the other stick is messed or could this possibly be a CPU problem? Should I switch sticks and test the other dimm now as well?

Note: When I removed both sticks and put them back in it did not restart fine unless I also shut off the PSU rocker switch in the back, is this normal or could that be resetting something else? Any help is appreciated.

Hi, it came with 2 of these installed: http://www.ncix.com/products/produ [...] &sku=18727
I have now tried both sticks separately and they both seem to work fine, I had run memtest for 3 hours 5 full sweeps i believe it was and I am really leaning towards your idea of low voltage. Unfortuantley the volts were already set and I can not see an option in the bios on how to change them. Is it possible that the motherboard will not let me set my own voltage because it is prettymuch a no frills motherboard?
This is the motherboard in it: http://www.ncix.com/products/produ [...] &sku=34425 I can not find anything on the internet/manual on how to change the voltage however.