Thanks for you answer Eileen. I am not sure any more if it is "just synovitis". Before it happened, back in May this year, I have changed to new bike, which had different geometry and different stress on my hands. Although I noticed that my hands hurt while I ride, I assumed it will go away. Late May I went for 3 week trip to EU, and that is when my hands (knuckles) became inflamed. Because of the timing I ruled out bike riding as a cause. In retrospect, it might have been delayed reaction, who knows.
After returning to Japan I have seen rheumi. He did ultrasound and made a comment about synovitis or early RA and gave me some topical oiment to use, which did not work. I saw him second time after a week and then he prescribed higher dose of pred, which worked. Third time when I have seen him ( early July) he was sure it was not RA, just synovitis. Since then I was able to reduce pred from 9mg to 4-5mg before symptoms started to return. The pain compared to PMR is more like annoyance, much lower intensity. Throughout this time I have been using my new bike too, and riding 1.5-2hours 2x per week. I do have to stretch hands during the ride, it is not really a pain, more like discomfort. Perhaps it is all because of the overuse.
The article you sent link for is interesting. I have tried the stretch exercise it suggests this morning and it seems it helps immediately.
The more I think about the problem, the more I am convinced that it is a combination of overuse and also blood supply restriction to the area due to PMR capillary inflammation. Hopefully I outgrow it over time.