Is my water pressure regulator broken?

Trying to determine if I need to replace my Water Pressure Regulator. I just replaced my 7 year old Expansion tank hoping that would stop my the pressure release valve (PRV) on my new (2 months old) water heater from dripping. The old bladder was blown but the new expansion tank has not stopped the leakage from the PRV. I have already adjusted the pressure on my Requlator and that did indeed drop the pressure in the house...you can see and feel that the water pressure is lower. After openinng and closing a faucet momentarily, the pressure meter hooked up to my hose bib measures about 70 psi. Here is where I'm stumped...the static water pressure builds over time and climbs as high 110 but after having the kitchen sink faucet run for a few seconds the water pressure tapers off significantly. And the flow of water from the sink faucet is good for just a moment and then the stream becomes very weak. So again the water pressure is both flowing and static is lower since adjusting the Regulator but why is the pressure still climbing so high...does that mean my regulator is blown?

It sounds as if your pressure reducing valve is bad. The pressure keeps building as it is slowly letting pressure pass.

A bad expansion tank would not affect the PRV. The expansion tank is just a temporary hold tank for the expansion that occurs when water is heated. If you tap the expansion tank, if it is good, it will ring sort of like a bell. If is just thuds, it is full of water and need replacing.