Are all of your VM's running on the same physical disks on which the ESXi system is installed? How many VMs?
I wonder if you have a VM with some runaway I/O processing causing the host system to become unresponsible (which might have been mitigated by reducing the CPU resources to the VM, as you mentioned earlier). 2 SATA drives doesn't offer very much in the way of available I/O or Kbps per second.