Try changing the Drill cycle to a G83 and make the Q value the same as the Z value. The G81 has always been glitchy. The G81 will also at times cause a stepper machine to loose steps on teh Z. ALSO with Mach3 it is a good practice to set G61 before ANY of the cycle calls. IF not, at times you can break drills,taps,reamers,etc. The cycles are SUPPOSE to set it internally but not all of the cycles actually do it. Be safe and switch it.
Just remember to set it back to G64 after the drill cycle is cancelled. G80 G64