Puzzled over this valve issue

I have a water heater which has a pressure relief valve, and a Thermal Expansion Control Valve (TECV). The TECV sets on the incoming cold water line just before the water heater and cold line is connected directly to water main.

See this
http://www.plumbersurplus.com/Prod/Apollo-Valves-78326RV-Water-Heater-Shutoff-Thermal-Expansion-Control-Valve-3-4/134852/Cat/218

Last month the TECV started to release water through the small attached pipe continuously. I hired a plumber who said you need a new TECV valve. It was hard to find but I managed to purchase one. He removed the old valve and put in the new one. It worked fine until this morning when it started releasing water again.

At this point neither I or the plumber know what to do. I do not think the new TECV is defective (or the old one). The main water pressure is 70 PSI as it has a pressure reduction valve. Could this be a bad water heater?

The pressure relief valve (150 PSI) on the water heater itself appears to work fine. It releases water when tested. It is a higher threshold than the TECV (80 PSI). The water heater is 13 years old and it is Rheem.

Turning hot water on for a couple of minutes stops TECV from releasing water until the pressure builds again.