However the video card, cpu, heatsink, power supply, ram and hard drive are all new.
Don't assume just because it is new that it is problem free. Also, are you SURE these components are compatable with your motherboard?
basically the only original part left is the motherboard. Could the motherboard be failing?
Sure it could be. But I think good troubleshooting calls for looking to other items first.
Think of this: You go into a room, turn on the light switch and light does not turn on. Do you fix this problem by tearing out the wiring and replacing it? No. Rather it is best to start at the manifistation of the problem and work back. So, in this case you should replace the bulb first. Then check for voltage at the bulb. Then check for voltage at the switch, and then check the switch, and continue working backwards until you find the culprit.
It seems in your case it would make sense to confirm it is not monitor related. ( by the sound of your description it does not sound monitor related, however. ) Then turn your attention to the video card. In most cases you would be able to remove the vid card and plug your monitor into the on-board video as a test. But in your case the motherboard does not have on-board video. Bummer. The easiest thing to try first is installing the newest drivers from the vid card's manufacterer's web site. If that does not cure the issues, then try a different vid card.
All of this assumes it is a hardware issue. Like kecik
suggests, this may be software related. Would it be hard to reinstall Windows? Or if this is a difficult solution for you, then play with the hardware first.
Tough to know for sure what the cause is at this point. Let us know the results of your testing.