Plumbing fixtures starve but water pressure 50 psi

We can’t run a shower along with any other fixture in the house or it will slow to a trickle.

I’ve measured the pressure with a gauge and we get ~110psi before the pressure-reducing valve and ~50psi after it. Apparently, that’s the recommended pressure.

I turned on the basement bathroom faucet and saw a ~10 psi drop in the internal pressure. Is that expected – or does the PRV keep a constant 50psi even under demand?

This is a 2 storey + basement 3800 sq ft house. I measured the pressure after the PRV using the tap for the washing machine. The house was built in 1986 – although there’s evidence it’s been replumbed since.

I’m now at a loss for what could be causing this.

It happens on both of the two working showers. For example, in the basement, if the toilet cistern is filling then the shower flow rate is about half. Upstairs, if anything else is running then that shower essentially stops.

If relevant, we also have a shower panel in another upstairs bathroom which is unusable because it is either hot or cold – nowhere in between.