7i96 PWM open loop spindle speed fluctuation

First time mesa user. I have a 7i96 driving 4 steppers with the 5th stepgen being used for PWM control of a cheap chinese VFD. Provlem is the VFD output is fluctuating quite a bit when not at min or max speed. If I measure the PWM output with a multimeter it appears stable. If I use a 10k pot to input the outage the VFD speed is steady and ontrollable. I have step+ and step- connected to the VLI and common terminals i use for the 10k pot. The 10k pot would be fed with 5v and the wiper feeding into the VLI terminal. The cable is about 3m long and heavily screened and earthed. This fluctuation occurs with anything running, even without the motor connected. Any thoughts on what may cause it?

Hal configuration shows the signals/pins to be stable.

What PWM frequency?
Does the VFD indicate it will take PWM?
If not you may need a RC filer

I'm pretty sure i saw a mention of 20k somewhere. Would that have been in the ini or hal files?

The VFD is a very cheap generic AT1, 2.2kw model from banggood that comes with next to no documentation. Ill cobble a filter together and see if that solves the issues as it certainly sounds like a good place to start. Thanks.

Typically PWM frequency setting would be in the hal file

I suspect he VFD expects a constant voltage so does not
do well with unfiltered PWM