mac pro freezing on sleep

This has happened to me twice so far.
I put my mac pro (2007) to sleep and left the room.
I come back a few hours later, the fan is running very high and mac pro is stuck. I have to hold the power button for it to shut off.

It restarted fine the first time, but this morning, when I came back and the fan was on again, I held the power button to shut it off. When it restarted, I got the "bong" but the fan was still running high and no display.

I shut if off again, waited 10 minutes, then it started fine.

I thought maybe the video card (X1900) but no screen flickers or any distortions. It's working fine now.

Is it just a sleep issue with snow leopard or something?

I have this issue on an 24" Aluminum iMac and have registered a case report on this issue (still under 3 yr Applecare). Mine started in December but I use Leopard and it occurs every am. They had me delete caches and this worked for 1 day.