[JohnnyL]

Could be lime and scale restricting flow somewhere and the water pump can't build up enough pressure at idle to overcome it fast enough. Especially at 300K. Radiators are notorius for clogging and restricing flow when they get old. Have you had the system flushed in recent years? Could be something as simple as air in the system too. If your recovery tank is empty, the system will draw air when it cools down. Might want to pull the cap while it's running and watch for circulation at idle. You'll at least purge air out doing that. Doesn't hurt to replace the stat. They are cheap.