Terrible disk I/O performance for VMWare guest on dual-core Opterons

[Fedora]

I've just switched from an MSI K8 MASTER2-FAR with dual Opteron 240s to
a Tyan S2877 with dual dual-core Opteron 265s. I'm running VMWare
Workstation 5.5.4 under FC6 and since the mobo/processor switch all the
guest OSes get terrible performance accessing the hosts disks. 

I'm using Samba on the FC6 host OS for a WinXP guest which mostly just
gives up after many seconds trying to read disk files. I've found lots
of postings on the net about time synchronisation problems using
dual-core Opterons caused by the TSCs not being synched between the
processors, and I've tried restricting the guests to just a single core
- but to no avail. I have not found any answers posted anywhere.

Has anyone encountered this problem? If so, does anyone have a solution?

TIA
Dick