Water Heaters - leaking from TPR valve- changed and still leaking

I have a 8 year old John Wood gas hot water heater that recently started leaking from the temperature pressure relief valve. I changed out the valve and it still is leaking. I have gas hot water baseboard heat.Since the home is 8 years old and I haven't had any other changes in the plumbing, and it has a pressure regulator on the main. Any suggestions? I do have an extrol expansion tank on the cold water feed into my boiler.I will test the pressure on the drain valve and then maybe could it be the thermostat? When I adjust the temperature on the blue temperature control dial ( bear with me) for hotter water, it seems like the dial is "stripped". Sorry I can't describe it better, but it feels like it should have more resistance when turning. Feels like its free wheeling....Any help would be appreciated.
Joe

The water from the tpr isn't that hot so its not too much heat ( thermostat?). Would after 9 years in my home with only one expansion tank ( before the cold water feed before the boiler)could I need another before the hot water heater ( in the cold feed?). I don't understand if that was the case. But I did lower the incoming pressure from the city side and its still dripping? Losing my mind... Help would be appreciated!

no you only need 1 but the expansion tank could be bad

I really appreciate the response. I checked the one expansion tank and it read about 14 lbs and no water came out only air. It sounds hollow...any suggestions?

well, I did put a tee into the cold water supply line for the water heater and installed another expansion tank. So far ( knock on wood) so good. seems like that resolved the problem. Thanks for the replies.