Insomniac P6T SE - problem going to sleep

I have a P6T SE that has been working fine since last November, but in June it decided not to go to sleep. It would go through all the normal motions to go to sleep, which I've watched it do hundreds of times. The last steps are hard drive light goes off, power LED starts to blink (indicating S3 sleep), and then in about 2 seconds the case lights and all the fans (CPU, power supply, etc.) turn off. When it acts up, this last step is immediately followed by the machine turning itself back on.

I ended up just turning the machine off all the time, since it just refused to sleep. Then it decided to quit booting. I would turn on the power, and the lights would go on, but then it would turn itself off again within 2 seconds.

I found this thread which described a similar problem, and I also had already thought about clearing the CMOS as a last resort. So I tried that, and everything began working again.

Now the problem has come back again. I cleared the CMOS again today, and it now seems to work, but I'm concerned the problem will come back. What could cause this? Is the motherboard going bad?

I've posted my questions in Asus' forums too in the thread mentioned above, but I'm curious if anyone here has some ideas that might help. I really need the system to sleep, since I use it as a HTPC, and it has to wake up in the middle of the night to record stuff I want to watch. It's certainly capable of doing this, and has been doing so for months -- my full hard drive is evidence of that  . It is only recently that it has become an insomniac.