KVM Host restart


I have several machines I am using to evaluate KVM. I have installed Centos5.3 on each of them. All 4 machines have kvm installed, but I have only created guest vm’s on two of them. Both machines that have guests on them restarted in the middle of the night. The others did not. Although I had created guest vms on these two machines, there were no guests running when I left last night.

I am new to KVM, and not sure how to go about diagnosing this problem. Any suggestions?