Answer
This question was answered on May 21, 2007.
Much of the information contained herein may have changed since posting.
This very well could be the motherboard once again but if you replaced the motherboard there could be some other issues involved here, one of which could be the power supply you currently have.
There is a chance that you are experiencing some overheating problems and may need to get a more powerful power supply or some extra fans put into your case to help cool off your hardware. There also could be a memory issue with conflicting RAM as well that will make it so your ciomputer will not function properly, or will shut off or freeze on occasion. This is a hard issue to diagnose on the forums but good luck to you and maybe a year from now we won't see this problem again :)
Kevin