American Standard, I believe 1032 approximately 7 years old. Cartridge was replaced after 'hammering' started, no more hammering but...
When the shower is running fully open, comfortably hot temp, there are no pressure issues; however when the tub component is utilized the water pressure will drop, after water temp gets comfortably hot, to a little more than a trickle. Conversely full cold water will not affect pressure.
I attempted running shower, ful and hot, without shower head, this caused the same drop in pressure. I am thinking that there is a relation to back pressure, but wondering how that could be. Filling a 6ft soaker tub takes a while this way.

Suspect that there is debris in the hot side port. Up to a certain volume on the hot side every thing is fine. When volume increase, which means the speed of the flow increases, the debris is moved forward to block or partially block the port.
Remove the cartridge and flush the hot side thoroughly.

Thank you, I'll give it a try this weekend.