It sounds like lost steps, which stepper-powered machines are prone to. If it was working well before, and considering it's a plasma cutter, the first thing to check for is stuff accumulating on the sliding surfaces, or the screws or gear racks if that's what propel it. Plasma cutting is a very messy process, and if these surfaces aren't well protected, the crud will stick to them and cause all sorts of problems. If that turns out to be the issue, look into bellows and other methods of shielding vulnerable parts. Has it got a water table? Those catch most of the flying debris, but not all.