WS 6.5.5 host peaks at 50% cpu utilization

Host: VMware Workstation 6.5.5 build-328052, running on XP w/SP3

Guest: XP w/SP3

When starting the guest, the host process vmware-vmx.exe peaks-and-holds at about 100%, which is good.  However, once the guest is running, when running a heavy-load process on the guest, the host process vmware-vmx.exe process peaks-and-holds at exactly <= 50% CPU.  The guest shows up to 100% CPU.  I’ve tried setting the guest with one and two processors, with lots or little memory, each time with no difference; the host’s process peaks at 50%.  Is there something I can do to increase the utilization of the host’s CPU?

Thanks,

Bob