I am having a problem with my water pressure. I have a well. When I turn the shower on, the water pressure is fine for about 4 minutes, then it goes down to nothing. I watched the pressure gauge on the tank and here is what I saw:

When the pressure was good, the pressure on the tank was between 40 and 42 lbs.
The pressure would slowly creep down to 38lbs.
As soon as it hit 38, it would immediately drop down 18 lbs and stay there until the water was shut off.
As soon as the shower was shut off, the pressure immediately jumped up to 40.

Any ideas what would cause this type of fluctuation?