The wire speed knob does more than control the wire speed. As the wire speed is increased the welding power must also increase and the circuitry to do this and keep things in sync is fairly complex. The fault is probably within that circuitry and the repair will require no small amount of skill and test equipment or a replacement circuit board.
Complex electronics can be fairly sensitive to high temperatures and voltage spikes and surges and apart from the natural sort that arrive along the mains electricity supply mig welding is an electrically noisy process that can be quite stressful on those electronics. 20 years of trouble-free operation is quite a good record...
Here are the most common causes of idle surge, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake
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