Not knowing what you are driving the relay with, I would guess you have a power or noise issue.
The Max-20 running at 20 amps will pull every available electron form a 120 VAC 20 Amp circuit.
If your cnc/control circuit is running from the same 120 VAC circuit it may be resetting from a momentary drop in supply voltage.
The MAX-20 did not have a machine interface as it is only a 20% duty cycle system.
It was never designed for machine cutting.
You can only cut for 2 min then wait 8 min for it to cool down.
The 120 outlet can also overheat if this is exceeded.
I think that at 10 amps it is 50% and 6 amps is 100%.
Not terribly productive.
I have a MAX-20 and it overheats and shuts down when you exceede the duty cycle.
I am on the road and do not have access to a manual so I dont remember what C8 does, but if you short out the trigger wires the machine will fire.