@Steve - thanks for the thoughts - I've already swapped things around (and remember that this machine will stop halfway through a recording but then work okay later - so the only realistic movement is heat-related), things are plugged into the motherboard USB ports (not the daughterboard, not a hub), and so on.
But certainly I'll have a think to see if I can systematically find what's happening. It must be something unique to my system... I'd blame the coax cable/signal if it were on one tuner only, but two? Of different technologies? Hmmm.
I might try a total rebuild to the new XBMCBuntu beta or similar - different kernel, different everything. What I'm desperately avoiding is making the switch to Myth or (shudder) one of the Windows back-ends. I think the work Adam and the team are doing here is great for the future of the lightest, easiest backend and I want to stick with that - if it works for me.
I even considered switching TVH to my NAS, but the prospect of trying to compile TBS drivers on that makes me shudder (plus the lack of grunt without swapping that out for a different unit).
Again, thanks for the thoughts, let me sit awhile in uffish thought.