Z looses it's place

I have had this happen 3 times in the past week. It is down approximately .060"
the tools I am using are 1/32 and a .020" ball mill.
My cuts are .003"
I have checked to see if anything is loose.
Someone suggested a encoder was bad :(
I had this happen one other time with the Y axis. It moved over 1" in the middle of facing off a piece of alum.
I reinstalled the Mach 3 as suggested by Taig.
I have suspected that it happened in the x also.
Any suggestions?