At a guess , If it is drawing a straight line without the plasma being on, and wobbly lines when it is, then I'd say that there is a significant electrical interference occurring.
I'd be checking the earth connections, making sure the motor cables are not running parallel to the Plasma torch line or earth lead anywhere.
Is there shielding in the motor cables ?
I'd also try again without the THC switched on. Try and isolate/eliminate one thing at a time.
Note : The proma THC instructions make a point of making sure the earth of the THC board is NOT shared with its power supply earth, it must be common with the computer/table earth.
trying to help,
cheers
Art