G76 z axis stuttering

Hello All
I am having a problem with G76 threading.
My lathe is a converted manual machine with no speed control (only belt changes).
I have a single hall pickup connected to phase a and index, which seems to work ok.
The problem I have is that the Z axis is moving in discreet steps once per revolution. Start, stop, start, stop etc.
The overall pitch is still correct but the feed is not constant.
What could be the cause.
Many thanks.
Adrian.

Is this a printer port setup? Using the software encoder?

Use encoder.N.position−interpolated instead of encoder.N.position when hooking it to motion.spindle−revs...

linuxcnc.org/docs/html/man/man9/encoder.9.html

Hello Skunkworks
Not only were you able to guess what setup I am using but nail the solution first time.
Probably really simple to most but not to me.
Many thanks, works like a dream now.
Adrian.

Yay!!! (even a broken clock is right twice a day....)