"Limit Switch Hit" message

[sawdust535]

Intermittently and randomly getting a "Limit Switch Hit" message and the tool stops moving (spindle stays on). When I hit resume, the file always continues to completion, but sometimes with a loud banging noise which literally shakes the machine. If I run the same file again, it sometimes runs fine. It does this with or without my dust collection running. Any ideas on what is causing this. I'm trying to solve this problem on my own and have read many threads on this subject, and it it looks like this problem can be caused by a number of things, from grounding problems, to faulty wiring. What causes that banging noise. I know that its not a physical thing because it happens when the tool is no where near the stops. It happens often when I use 2D offset.
I have a PRS Alpha BT32 with a spindle.