Follow up. The airplane has been with the shop for a week. Various things were done, including new plugs and a clean up of the fuel injectors. The prop was checked.
I flew it yesterday, and checked the prop myself, as well as the mixture and a few other things that might have had a bearing on what I was seeing, but it looked good.
During the flight it was perfectly normal, all instrument values within limits. On final around 80 knots with idle power the rpm was 1600 and at 65 knots it was 1300, quite respectable and acceptable.
So whatever was wrong with it now seems to have scampered off to infect some other person's airplane.
Thanks again for the help.