Problem with 2003 Rheem Natural Gas Water Heater 40-gallon

Symptoms:
1. Dripping water from t&p valve
2. Hear air hissing from t&p valve,
3. Low water pressure from hot water heater (cold water pressure is fine
into the water heater - has been checked)
4. Tonight air was causing the hot water to sputter through the pipes/faucet
5. Hear gurgling and popping noises from water heater when it is heating
water

I am going to flush the water heater later today, but I don't know if that
is going to increase the hot water pressure. I finally got the t&p valve to
quit leaking and I don't hear air hissing from it anymore. I also turned the
water heater temperature down to 120 - it was set at 140.

What else should I do or look for? With the move, I don't really want to
have to call a plumber if it is possible for me to do the work myself.