Why does my electric hot water heater keep popping the popoff valve ? I have already installed a new one and it still does it periodicaly .

You need to check the tempurature of the hot water thermostat setting if it is to high than it will open.

How old is it, I have seen in those that are getting old, with a lot of sluge in them that this can be an issue.

Also as noted what is the temp it is set at, and is the temp turning off at the correct temp.

Is the PRV popping or just periodically dripping?

If you are on city water, most likely you have a backflow preventer or a pressure regulator that keeps water from flowing back the inlet as the water heats and expands. It has to go some where and would burst the tank or lines without the PRV. The solution is an expansion tank. Water utilities have been know to come through and install back flow preventers without telling the homeowner they must install an expansion tank.