Hi,
so the welder is not defective,
but you want to fix a design flaw.
I think it´s much more difficult, because you need to find the path for the whole control loop.... and find out why there are the fluctuations come from and how to fix them.
It may be a timing problem, maybe noise is intrduced, maybe a software problem, a hardware problem, a control loop adjustment ...
***
If you don´t have a shematic and tools to follow the signal path.... it´s just a trial and error method ... maybe with the result to really kill some devices.
Sadly I guess I can´t help you on this.
Good luck
Klaus