clipboard fails to travel between Host and VM sometimes

I sometimes find that when I highlight and copy some text inside a VM,
when outside the VM, I can't paste it. Same happens in reverse too.

It doesn't happen very often, maybe once a week or so. But when it's
happening it persists and won't recover. I don't recognize any
particular event that causes it to happen. Basically the only things I'm
doing in the VM is running Trillian for IM, Thunderbird for email, and
occasionally Firefox, but I'm running all three when the problems is
occurring and also all the time when it's not happening.

Host = XP home SP 3 with 3GB memory, Core 2 Duo, 2.2GHz
VM = XP home SP 3 with 768MB memory allocated to it, no hardware
virtualization.

There's no special or unusual clipboard apps running on the VM or the
host, it's completely normal and works 99% of the time.

Virtual PC is version 6.0.210.0 (VPC 2007)