Does my motherboard need to be replaced?

I am having an issue where my computer will not turn on. I did this build over a year ago. I recently replaced the video card and went to a larger power supply. I was playing Skyrim the other night and the screen just went black and seemed like the computer shut off, but the fans and case lights were still running. If I tried turning the computer on I would just get the fans and case lights but motherboard would not beep and there was no boot indication on the monitor. I opened up the case and noticed there was some dust build up in my CPU fan/heat sink and it felt a bit warm to the touch. I cleaned it out along with disconnecting the power cord for a bit. I let it sit for about 30 mins and came back and it booted up without issue. Well the next night it did the same thing and the screen went black while I was just web surfing. Disconnecting the power cord made no difference this time. I am thinking my mobo may have died on me, but really not sure. Any thought or ways to confirm this before purchasing a new board? Here are my specs... Nothing is OC

Win 7 64
Gigabyte MA785GM-US2H AMD785G Socket AM2+ MB
Kingston HyperX 2GB 240-Pin DDR2 SDRAM DDR2 1066 (6 GB Total : 3 x 2GB)
AMD Phenom II X4 945 3.0Ghz 6MB AM3
HD Radeon 6950 2GB
Diablotek 850W Modular Power Supply