Thanks, Noel, I tried removing the rollingTierX lines and changed snapshot.numRollingTiers to 0 rather than 3

Now my guest OS is running glacially slowly -- maybe 10 or 15 minutes to bring up the ctrl-alt-delete login prompt, and keystrokes take forever to register.

Any suggestions?

I might have removed all of the entries related to rollingTiers and/or snapshots. At this point, you might consider creating a nice clean VM, and then move (a copy of) the VMDK over. Mind you, I'd cleanup the state of the VMDK first, e.g., remove snapshots, after backing it up.