I moved my well tank to the crawl space. I replumbed everything with no problems. Turned the power on, the pump and lines are hammering. Pressure gauge reads 100 and relief valve trips. What is wrong?

Check the little pipe leading to your switch. See if it's clogged with crud. That will cause the switch to respond slowly. If that seems OK, then turn the large center nut down ccw several turns until you get to a 50# or so cutoff. 100# is really high and approached dangerously high.

People sometimes ask why well systems should have PRV's. Your case is an excellent illustration of why.

The pressure has stabilized around 50psi, but the water pressure lowers after running the water. There is air in the lines even after we ran all the faucets. The pump builds the pressure back up, but still little or no water. We just replaced the pump 3 days ago and decided to move the tank. When we restarted the well, the water won't stay pressurized after a few seconds. Tank pressure is 45psi, pressure gauge at pump reads 45-50. The pressure switch sparks when pump is running. Any other suggestions are welcomed.

Describe this "sparking" you referred to. Does it spark the entire time the pump is running, or just when the pump cuts on or off?

The problem has been solved. The tank had to be close to the tank, so we moved the tank/bladder back to original position. Thanks everyone.