water pressure up and down

My water pressure goes up and down quickly at the faucet. Is my pressure tank failing or something else? My pressure guage goes up to 40psi then quickly goes down to 20 then the pressure switch kicks in and back to 40 every 6-10 sec. What's up?

If it is doing that when you aren't using water, you have a serious leak. Shut the water off to the house and if the pressure falls, the leak is on the well side of the pressure tank. If not it is on the house side.

If you mean while you use water, the pressure tank needs the correct air pressure in it with no water in the tank. That is 19-18 lbs for 20/40 psi. The bladder could be broken or an automatic air volume control on an air over water type tank isn't working.

The pressure gauge does not change when no faucets are being used. The guage drops to 20psi within seconds then quickly back to 40psi and back to 20psi in about 6 sec. when the water is running. The water surges back and forth at the faucet.

Does your tank have a tire type valve near the top, if so, turn off the pump, drain the water out of the tank, measure the pressure at that tire valve, if it is not 18 psi, add air until it is. If after adding air it continues to short cycle then you probably need a new tank.

Rancher