I have a water heater that is at least 8 years old. The T & P valve was intermitantly leaking a little. I replaced it with a new one (T & P only) and the problem persists. The temp is set midrange on the water heater. It seems to leak a little only when water is being used upstairs (shower, dishwasher, etc) The water heater itself is not leaking, is it going bad or do I have a problem somewhere else?

Did you test the house pressure? In many instances, the T&P is fine, the PRV is bad or set too high. Get a pressure gauge and test the tank pressure at the heater. Pressure should not exceed 60 #.

Several things come to mind.
1- the house PSI is too high **OR**
2- you need to install a expansion tank on the water heater.
Good luck, Tom

You got good advice above from our other Experts.

FYI: T&P valve is a safety device. It is designed to leak if your water pressure is too high or water temperature is too high, too. Locate the problem and fix it before replacing T&P valve. Once the problem is fixed, install new T&P valve. Only that way it will stop leaking.

Hope that helps

Milo