Hot water pressure decreasing

[cdtech20059]

Hello I have a state gas 50 gallon hot water heater. 5 years old. Pressure at the tank Spigot is 65psi. Outdoor spigot is 80psi. When I turn on two or more hot water side faucets or showers at same time pressure decreases by 7-10 psi. The more I turn on the hot water tank pressure decreases. So if 4 things running it goes to around 39psi. Is this normal? My expansion tanks is good and has 63psi.
Maybe a clog in cold water inlet? No idea what it could be. Cold water throughout house is constant good pressure. Thank you.

 

[breplum]

Does sound like flow restriction, possibly the cold inlet or hot outlet on the WH.
There are a few versions of dielectric nipples that use floating ball or rubber flap that can deteriorate to cause this.
Start by replacing the dielectric nipples.