I've actually been afraid to post my results in here until I was *absolutely* sure that my problems have gone away, but it's now been long enough that I think I can say I no longer suffer from this problem in VMC.
Unfortunately, the way I got rid of it was to open the machine, remove my 2 Hauppauge PVR-250 cards that have been so reliable for so long, and replace them with 2 Hauppauge PVR-1600 cards that have both ATSC and NTSC tuners in them. Without changing another thing about my machine other than loading new drivers for the new cards, my recording problem went away.
This is good information I suppose in the sense that it does seem to verify Microsoft's claim of the source being in the tuner driver itself, but there's no question that folks with the HDHomeRun tuners or Hauppauge PVR-250 tuners are effected. (and the PVR-250 has been one of the "certfied" cards forever I thought)
Matthew