There is an index setting in Config> General Logic Configuration called Index Debounce.
The input value represents how manny times Mach3 needs to read and see a the index trigger.
You will need to try different values to suite your setup. I use a value of 10, some can get away with 1,
and others required greater numbers. If too high the Z axis will just sit there a long time.
The more accurate and steady the actual rpm the better. If it's all over the place you will never do accurate
threading and have problems. Plus or minus 1 or varying some by say 1 to 1/2 rpm should be ok, BUT,
get it as accurate and steady as possible. Then do some testing.
Get the index working correctly first and suggest you read / study THREADING ON THE LATHE-MACH3 TURN
which you can find in Members Doc's or also in the directory where you installed Mach3 if they are still including it in the installation of Mach3.
RICH