System reboot when use kvm-virtmanager

[lucho115]

hi, iam using kvm with virtmanager since a while without problems (and also virtualbox) but now when i start a virtual manchine the host system reboots (virtualbox still run without problems), i dont know how to fix it. My system is an amd phenom x6 1100T, 16gb of ram (amd 1600 cas9), 1 tb wd blach edition sata3, amd HD5650 1gb ddr3, an the SO is ubuntu 12.04 64 bits.
I have to say that boot with the ubuntu 12.04 64bits live-dvd, install kvm, and try to use one of the virtual manchines and the system reboot like with my installed system, 2 weeks ago i have no problems, maybe its a hardware problem, but virtualbox and all the others programs runs ok, the only that have problems is kvm. I test the mem with memtest and in 10 passed have no erros.
i will waiting for yours response.
thanks and sorry abouy my english.
bye