DDR3 RAM?
use Memtest x86 and verify your RAM is good
Xeon 55xx or higher CPU?
What is your storage? Single disk? take it offline and use smartmontools to verify the health of the drive, all smart tests must pass and you need to do a -t short and a -t long on the disk
How much RAM in the host?
How much ram to each guest?
How many CPU's?
How many vCPU's?
Do you have Hyper-threading on?
is the machine over heating?