.649 and .667 kernels and random lockups

Hi Everyone, I've upgraded two machines to fc3rc5(ish) and then upgraded again to latest rawhide. I've been running the .649 and .667 kernels on these two systems. One is a dell dimension 2400 - p4 2.8ghz the other is an ibm x23 thinkpad - p3-866mhz. Both of them will lockup if they sit idle for a while. It's a hard lock up, I can't free them of it and I have no idea what causes it. Is anyone else seeing this, too? Thanks -sv