Square D (30-50) cuts in intermitently. Installed new switch. Same problem.

What kind of pump, above ground or submersible?
What does pressure gauge show when pump kicks on?
Do you have a check valve in the system? If so, is it between the switch and the tank or is it between the switch and the well?

Be sure to also replace the little pipe leading to the switch. Clogs easily.

Is the switch within three or four feet of the tank? If not, it needs to be.

Might add that you need to be sure the tank is pressurized correctly. With system off and pressure drained to zero, tank pressure should be two pounds BELOW the cut in pressure. This is not likely your problem, but it is easy to do and I have seen it cause strange problems that I would not have thought it would be connected with.