The Comcast technician just left.
After quickly deciding the cable signal strength was marginal, he installed at 15dB amplifier outside the house (drilling a hole through the wall to get power from a plug-in transformer).
The amp seemed to fix the macro-blocking we saw on ESPN-HD this morning (which wasn't even showing last night)
So I have my fingers crossed that tonight the HD channels that were missing will still be there. I have to say though, I can't think of a reason why channels would come and go depending on the time of day...
By the way, this tech (I didn't get his name) really seemed to know what he was about. He even fixed a problem with my RCA cable modem I didn't know about (intermittent contacts on the power connector). Nice going, Comcast!