Tracking down the interference can be quite difficult. Try running the wires away from the stepper wires.
You can increase the input debounce in either config>logic or state. try 2000 and gradually increase. If it's too high, you'll get a delay when the switch trips, but it should get them working.
with 1/4-20, you'll have a hard time going faster than 30ipm. You need to switch to 1/2-10 or something for more speed.