Domestic hot water pressure

[pattypape]

We occasionally lose hot water pressure at all faucets,
The cold water lines retain pressure.
This occurs when on City water hook-up or on Fresh water tank pump.
This is intermittent and eventually hot water pressure returns.

Is there a check valve in the hot water line???
Or is there a hot water mixing fitting to help lower the very hot water
temperature created, when on coolant motor-aid hot water???

Any suggestions are welcome.

Bill 88 FC Michigan