Desktop won't sleep

I have a custom built (by me) desktop with an Asus P5Q SE mobo, Intel E7200, etc. The hardware shouldn't matter that much but my problem is, it won't sleep properly. Here's what happens:

1. I click sleep
2. It goes to sleep
3. Like 10 seconds later it starts back up but with the fans running 100% and just hangs like that, nothing on the monitor and I have to do a hard shutdown to get it back.
4. When I boot it back up I'm greeted with an "Overclocking failed" message and I can either load the default values or enter setup
5. I enter the setup
6. Change nothing
7. Exit setup
8. It starts normally

The CPU is overclocked to 3.5Ghz but is most definitely stable, I game on it all the time and it was originally built like 2-3 years ago.

The two main causes of unexpected waking are one of the input devices giving a false signal, or Wake-on-Lan being activated. The symptoms you describe sound more like the CPU is overheating on shutdown, waking to get the fans going to reduce temperature and failing the task. It may be something very different.

I've had problems with it in the past so the keyboard, mouse, and LAN wake capabilities are all disabled.