We live in a house where share a well with 3 other houses.what would cause the water pressure to keep going up over 100 psi on my gages . I replaced the pressure tank,the cut off switch 30/50 and all the pipes coming in from the well.where do I look next to solve this problem?

The part that determines the cut off is the pump control box. The replacement box should have shut the power off to the pump when it hit 50PSI. If it didn't, and if this is the only control box that's connected to the system, then the control box has to be faulty. Having said that and since this is a new control I would check the tubing that connects the pump to the control box for a clog or blockage. If this line's clogged it would cause the system you describe. Good luck, Tom