If I understand your problem, it is similar to one I had with my 355 a
few years ago. All of your problems involves the foot pedal? You're
using the pedal while stick welding?

My problem was damage to internal wires in the foot pedal cable from
being squashed one time too many. It took a while to figure out because
I did not expect that it could shut off the welder.

I put a continuity meter on both ends of each wire and wiggled the
cable and found the locations of the shorts. Opened it up those spots,
wrapped tape around the problem wires and has worked ever since.