The Pot that is connected to the voltage knob, If its bad, it could be faulting out with use because of heat that its generating internally.Like the voltage setting itself? or is there an internal pot that can be adjusted?
I'd venture to guess the machine is about 10 years old? I got it for basically free from a guy that knew next to nothing about welding and let me have it for scrap value. He was having no luck with it, and said he suspected the board for what that's worth.
I cleaned up the ground cable contacts and cleared a spiders nest out where the gas connection is made and it's getting better, aside from this problem.
I'll see how the voltage is looking. i'm running it from an outlet I installed right below the service panel, then through a 10/3 x 25' drop cord i made.
The steel is pretty dirty.. I'll try to clean it up and check the rest of the stuff you guys mentioned and report back.