I have a Miller 252 mig. The last couple of times using it it has refused to strike an arc. The wire will feed and the gas will flow but it will not strike an arc. This has only happened after using the welder for a while, not immediately at startup. And it doesn't quit while running a bead. I thought it could be at the top of its duty cycle but I really doubt that given the type of welding I do - and the fan hasn't even kicked on! Shutting it off and on has no affect. After about 5 minutes it will work fine again. I considered a bad ground but checking all connections disproved that. Since the wire and gas feed I can't see it being the gun switch. I changed the tip fwiw but that didn't help. Wire is not rusty. Any thoughts? Machine is like new with very low hours but is off warranty.