Looking for suggestions: I have a 2 years old CNCRP Pro 4848 with stock plug and play Nema34 system and spindle.

I occasionally have a problem when changing bits. When I hit hit auto zero button in Mach3, the spindle slowly raises, instead of lowering. Sometimes the spindle will slowly lower but stop before the bit hits the plate, acting like it does when operating properly but not actually touching. If I shut down Mach3 and reinitialize it works. I just hate doing that in the middle of a job.

Any thoughts?