So, the heater worked great for a week or so, then did its weird thing again. I bled some some more air out of it, and it again it worked as it should. Then, suddenly about a week later, the SES started flashing: misfires on 4 and 8, O2 sensors, etc (codes 0304, 0308, 1171, 1174, 1300). Started it up 2 hours later, no SES, but it ran like crap. This was on a Friday. Monday I took it to Taylor Congleton, based on recommendations and that I just happened to be in Colchester that day: He smelled the problem from 20 feet away, even his dogs cringed, "You need some new head gaskets," he said.
I had never overheated, never lost any coolant, oil looked ok; then it all went to hell in a hand basket.
So, there ya go, another indicator of failing head gaskets: random heater problems.