computer will not boot

My computer usually won't boot after it's been off for a number of hours. Only the fans and lights come on. It stops at post code C1. Usually there is no beep code, but occasionally it will beep one long, pause, one long, pause, etc. When it doesn't start, turning off the computer requires holding down the button for 30 seconds, where normally a quick tap would suffice. Clearing the CMOS, or removing and reinstallng the ram, or pure luck, will get it started. Once started, it can be restarted as often as I want, with no problem. It also runs perfectly once started. But then in the morning, after it's been off all night, it again won't boot up.

The computer is one year old, and has run with no problems the entire time.

Abit IP35 Pro
Intel Core 2 Duo E6750 2.66GHz
Cooler Master Real Power Pro 650 W
Crucial Ballistix 4 GB (4x1 GB) DDR2 SDRAM DDR2 800
BFG Nvidia GeForce 8800 GT
Creative Sound Blaster X-Fi XtremeGamer
dual booted with:
Vista 64-bit Home
XP Home

At wit's end, I took it to a computer repair place. The computer techs gave up. Memory and power supply have been tested. All okay. It's hard to diagnose, because you have to let it sit at least one night for each step. Even then, it randomly does boot, and then it won't the next time.

Does anyone have an idea?

Welcome to HWF :)

According to the series of long beeps, that indicates a problem with the RAM. But you say that the memory has been tested and also the power supply, so it sounds like a problem with the motherboard itself. But first remove all RAM sticks, except one. Look if the problem comes back. If so, try a different stick. If the problem comes back again, then it's possibly a faulty motherboard.

Thanks so much. It became very obvious that there was one bad stick of RAM when I only inserted one at a time. The computer repair person I had taken it to only used MemTest 86 for testing the memory. This morning the computer booted up minus the bad stick of RAM with no problem.
Karen