SOLVED! Mostly. I tried a couple of other things, such as remove & reseat components, and various BIOS settings. What seems to work best is to use the PS fan connection lead to the mobo PS fan connection socket. I had thought the fan lead from the PS was supposed to connect to another fan, like a case fan. But instead it looks like a signal line to the mobo so the mobo can detect or monitor the PS fan.
Even though the PS fan speed wasn't the problem, having the mobo "aware" of the PS fan seems to let it reduce the cpu fan speed when the cpu cools down. It still takes a while.
Thanks for the ideas!