i would check for over-use of thread sealant, it can cause a blockage: slow water and/or not allow it to reach expansion tank during heating cycle. maybe an old valve in-line not open all the way? thermal expansion is a real thing, when you have a closed water system ( most city water applications) it is required to have an expansion tank on cold side of water heater, also it will drip out of t&p valve when not present. over-soldered copper fittings, crimped/damaged pex, sediment in piping, can all do the same thing. gas tank water heaters are suppose to have a minimum of 18" of copper going to and coming from the water heater. how the pressure if you hook garden hose to water heater drain? or remove outlet(hot side) near water heater before it goes to any fixtures? can also remove inlet side and check pressure there. based on experience you probably have a build up of teflon at either the cold or hot heat stop nipple attached to water heater (probably cold side). teflon tape only works one-way, wrap it in the direction of thread like your tightening ONLY! hope this helps, if not post a pic up.