It sounds like your problem isn't the stepper drivers, it's getting the signals to the stepper drivers that's the problem.

Plasma cutters are well known for emitting a lot of emi, so you have to make sure all the signal cables are shielded with the ends properly terminated and earthed...but it sounds like you've already done that?

Perhaps post some pictures of your control box so we can see what you've got and if there's anything obvious?

What motors are you using? You might be able to run them all from the 36V PSU.