When I run the water in the bath tub it runs for about 30 seconds then stops.It starts again and about 20 seconds. It cycles like this until I stop the water. If I use the sinks or toilets it seems to be fine. I watched the pressure gauge and it goes to 30 then right down to 0. And then cycles the same way until I stop the water flow,'Tahks,
Al

What kind of tub faucet do you have ? Single handle, two/three handle ?

Single handle

There is most likely something wrong with your cartridge.

Turn water off, remove handle and trim, remove cartridge, inspect it, clean it and reinstall it. See if that works. If the problem continues, you will have to install new cartridge...

Is that an old faucet or newer model ?

The faucet is about 20 years old. I tried it in the second bathroom and it does the same thing.

So let me recap: sinks and toilets work just fine. Only two tub/shower faucets in your house have this problem. Correct ?

You are on a private well, right? I suspect your pump is cycling but not soon enough to prevent loss of pressure. Have someone try to fill the tub while you watch the pressure gauge on the tank, tell me at what point the pumps kicks back in, what psi.

So let me recap: sinks and toilets work just fine. Only two tub/shower faucets in your house have this problem. Correct ?

Yes

So let me recap: sinks and toilets work just fine. Only two tub/shower faucets in your house have this problem. Correct ?

Yes that is right

When the tub is filling it only goes to 30 pounds then drops to 0. Then it goes back to 30 then down again,

You are on a well. What kind of pump, surface or submersible? What kind of tank bladder or pressure tank? What are your on and off pressure settings?