Mach3 Lathe Threading Struggles

[mondrota]

Welcome and thanks for reading my post.
So basically I'm trying to cut some threads on my Boxford Model B that I converted to CNC and am using Mach3 to control it.
The spindle is driven by a VFD, controlled by a breakout board 0-10v signal.
I have a spindle feedback system installed and it's fairly steady (the fluctuation is about 3rpm).
If I have the Closed Loop Spindle Control ticked, the threading program runs fine but it seems like every pass is slightly out in relation to the previous one and the final thread is all messed up.
If I have the Closed Loop Spindle Control unchecked, the program starts to run but pauses just before it is about to cut the thread. Is it waiting for the RPM to stabilise?
If I have Use Spindle Feedback In Sync Mode checked, the same thing happens as above.
Please have a look at the attached pictures, I hope they will help you guys diagnose the problem.
Any help would be much appreciated.