Hey John,
this sounds like the z plunge issue. Maybe → this and → this will help you.
There exist many issues around the Z axis not doing what it should. See Z plunge issue. Some are caused by bugs in newer firmware, some by cable breaks in the ‘curly’ Z stepper cable, some are caused by EMI due to the use of non-shielded stepper cables on the machine, and some by bit slipping due to the mediocre ability of the hand trim router’s collet to hold bits (in comparison to ER collets on spindles and 43 mm “Euro” milling motors).