Prpbly it's debris in the valve. The valves are cheap so it's a lot easier to replace than try and find the debris. You'll first have to replace the shut off valve which may require a torch and no lead solder. The pipes need to be dry for the solder to set up properly. I just went thru the same issue with a brand new Kohler tolet I installed, worked for a while then started to continually run, changed the valve and it's fine now. Mike