Vista - Parallels Hangs at 100%, can't kill parallels process, must pull power cord

Dear Forum,

I've been running into some strange, crippling crashes with my Windows Vista guest OS and I was wondering if anybody has had crashes similar to mine. I have not yet been able to reproduce the bug, as it happens unpredictable-- I've had the same sort of crash while I'm typing away in Visual Studio, when the guest OS is idle and nothing is running, and also when Vista is (almost done) booting. (I've also had a few blue screens when resuming the OS, but those crashes seem unrelated and don't crash my entire system as I describe below).

The problem is essentially that the guest OS hangs and brings down my entire computer. First the guest OS will hang using 100% CPU and all attempts to pause, suspend, or even stop/reset the guest OS will fail. I can quit the parallels GUI while it's hanging in the middle of suspending/stopping the guest OS, but there will still be a process (something like "pvm_app") running in the background using 100% CPU. The major problem is that even "sudo kill" in the terminal will not kill the runaway process; in fact, doing "sudo kill" locks up my entire system after executing the command. If I'm lucky to have a terminal window open after doing "sudo kill," I cannot even do "top" or "ps" -- it's like Mac OS's process management system is completely broken at this point. Furthermore "sudo shutdown -r now" will print a "system shutting down message" but fails to reboot the system. The only way to restart the machine is by holding down the power button.

Has anybody experienced these sorts of crashes? I had a couple of these incidents a few weeks ago, but Vista has otherwise been stable. It's only today that I've now experienced more than five crashes in a row. Windows Update has run a couple of times in the last couple weeks, but the crashes have been happening before and after all updates.


Specs:
Vista Business on Parallels 4.0.3844
Mac Pro quad 3.0ghz, 8gb RAM

Thanks for anybody's advice or stories!
Cheers,
-Paul