My guess would be that you have an air flow restriction or leak....somewhere between the inlet air fitting and the torch nozzle. A tip off to this is that you say it works better at 80 Amps.
I would look carefully at the entire air flow system, dissasemble and clean or replace the filter element and ensure the internal moisture trap is operational.
Make sure all air fittings in the torch leads and on the back of the torch body are tight, ensure that the o-ring on the torch body is in good condition, replace if any doubt.
If you don't find anything along these lines, it is entirely possible that there is something leaking or restrictive in the torch lead or the torch body itself. The best way to troubleshoot this would be to swap out the entire torch and lead assembly....and see if the problem goes away......do you know someone with the same setup, or does your local distributor have one?
I am assuming that you are using Hypertherm consumables.....and that you have tried changing things such as the retaining cap (they last a long time, but when they go bad they will exhibit symptoms such as you describe.
Keep me posted!
Jim Colt Hypertherm