1) The Plasma cutter is shutting itself off, or believes it is receiving an off command (start loop sees open contacts) and immediately re-fires the torch. In automatic mode this would result in the torch shutting off, then moving away from the material in large steps (reading low voltage), then when the torch attempts to fire (spike in voltage) it will get closer to the material. In Manual mode the torch would stay near the material, shut off, then re-ignite.
2) The AVHC believes it is receiving an off signal (contacts reading open from Plasma Interface box) and therefore shuts the torch off and retracts to the top. In Automatic or Manual mode the torch would shut off, retract to the top, and then if it got an on signal again quickly it would come down sense the material and fire.
3) The Plasma Interface box relay is not giving a 100% contact closure, or is not receiving 5 volts DC consistently from the Signal Generator. If this 5v is lost the relay will open shutting off the AVHC resulting in the same occurrences as #2.
If the problem happens only once out of every 100 times the machine cuts, it would be very hard to diagnose which of these it is, but hopefully explaining the actions of what the system does when a certain thing happens might help you narrow the problem down.
Also any individual component of the AVHC system is available for purchase at any time, from the control Box to the Cables, to individual nuts and bolts. However spending the time in shipping and upfront cost on replacement parts seems like a poor way to diagnose a problem.