OK, solved the problem by repositioning the strap (per Polar customer service instructions). They suggested I rotate the strap by 90 degrees so that the transmitter is on the left side of my chest and the two electrodes are front and back on the left side of my chest. Sure enough, no more interrupted readings on the bike. So I guess what was happening in the original strap position was that when I dropped into the aero position the strap was moving somehow, just enough, to interrupt the signals to the electrodes. Weird.
Phew, can now get consistent HR readings. Now, if only the Polar Power readings weren’t so volatile. It’s better than nothing, and by far the least expensive system, but if I had an extra 1500 I would go with a proper power meter, probably an Ergomo. Sigh.