First, get an inexpensive water pressure gauge from any hardware store or big box. It will fit on hose threads, so you can put it on the bottom heater drain valve or an outside spigot to check the actual water pressure at your house. This will verify if your pressure reducing valve is working properly. If it isn't, the PRV may need to be replaced if you have adjusted it properly and it isn't functioning correctly.
If you have a PRV, then you always need an expansion tank because the PRV creates a "closed" system.
Next, carefully open and close your heater T&P valve quickly to flush out any debris that may be holding it open causing it to "leak". That could be your only problem if the pressure is where it should be.