Why does my gauge on the tank go from 55 down to 35 then back up.again when ever I run my water?

Well, it's supposed to. You use water and the pressure goes down, and then the pump kicks on and the pressure goes up. The question is, how long does it take. It should be a relatively long process in both directions, meaning a few minutes in both directions. If this is happening fairly rapidly, as in a few seconds each way, then that's trouble. There is a little air valve at the top of the tank, like the ones found on car tires. Push the valve stem for a few seconds. You should get air only. If you get water, then the tank is bad and needs to be replaced.