Since you have good running hot water every else I think it is in your tubs hot water valve. Is it a double or single handle faucet? Do they have shut off valves that you can see? If you can open these all the way for a start. I dont think that it is a pipe problem but in that particular valve some where. If its the original faucet you may have to get a new one. A couple of other things come to mind. Air in your line or clogged screen strainer. Open up ALL your hot water faucets in your house and let them run for awhile and see if it changes. I wish you good luck now Father Bronze. This exhausts all of my ideas for you. Let me know how this turns out oh scalded one
------------------
*\Conan/*