This is an old house(early 70's) with an original tub/shower enclosure. When showering, for the first 2 minutes everything is fine. Then the water pressure drops to a trickle. Temperature doesn't change just the pressure. Shut the shower off at the tub faucet and start it over, the pressure is fine for another 2 minutes. Problem is not the shower head, we use a detachable hose/sprayer. Is the problem in the shower control on the faucet where it closes off partially after two minutes or somewhere else I'd rather not know about?

If your plumbing is metal, maybe the rust is closing it up inside. That's what happened to the place where I used to work. They didn't replace the pipes though, they hired a company who cleans these pipes by applying a lot of water pressure to them and so releasing some of the rust, I think. But that was a big building and it was cost efficient to do it that way, in a house however, maybe replacing the pipes would be better, I hope your problem is simpler than this though, good luck.