G76 incorrect pitch

Ver 2.8
AC Servo with built in encoder using A/B & Z signals

I'm having issues with the thread pitch whilst using G76

Every thing else works fine with regards to positioning. For example, a 10mm cut in Z axis is exactly 10mm, and the same with regard to X. 
However, I cannot obtain the correct thread pitch, or maintain it if a different RPM is used.
For example,
If I wish to cut a 1.5mm pitch thread at 1000rpm, then the resulting pitch is much finer than 1.5mm
If I try again, but with a lower rpm, say 500, then the pitch is much more coarse.
The higher the RPM, then the finer the resultant pitch.
The start point of the thread is always maintained.
Encoder Position via HALshow indicates it is counting correctly per one rpm of motion.
Could this be a Latency problem?

 

Could you tell us more about the hardware you are using?
Is there gearing between the motor and spindle?

Posting your config files could help too.

Gearing is 1:1
Encoder Position counts correctly after testing with multiple rotations.
Files attached

To add the the above.
Ive run three tests with the same G76 program, each with differant RPM speeds but all with the same F1.5 pitch

The parts were measured on a Digital Shadowgraph:

1000 rpm = 0.8 Pitch
500 rpm = 1.5 Pitch
300 rpm = 1.5 Pitch

It would appear that it can't keep up after a certain RPM?

I adjusted:
Servo to 1000000
Base to 100000
And I get the Latency Error. The lower I set the Base, the more frequent the error message.

If I set Base higher to 110000, then I get 'Joint 1 following error'

When I run 'Latency Test' via Terminal, I see Base Jitter around 10,5000
Do I have an under performing PC ?

I have 2.8.2 installed

uname -a revealed:
4.19.0-14-rt-amd64 #1 SMP PREEMPT RT DEBIAN 4.19.171-2

Lantency Test revealed:
Running  glxgears + web browser + pdf viewer

Test 1 (on board graphics)
SERVO JITTER 39395
BASE JITTER 58168
INTERVAL MAX SERVO 1039395
INTERVAL MAX BASE 83168

Test 2 (PCIe 16 Graphics Card)
SERVO JITTER 66909
BASE JITTER 54364
INTERVAL MAX SERVO 1066909
INTERVAL MAX BASE 79364

Also, I have a recent problem. Ever since our new router arrived, I cannot connect the CNC Lathe to the network for updates

I seem to have cured the Latency error and Joint following problem by re-setting my driver boards to quarter turn, and adjusting the SCALE from 800 to 400.
However, when I feed with G1 the axis movement has become very slow. Rapids appear the same, and positioning is correct and accurate. What have I done wrong?
Also I have noticed if for example a set the feed to F0.25, then GMOCCAPY shows only 0.2 and so on.

.INI attached