Like Tattoo3 said about the feed wire. this is where I was thinking when you were describing your problem. When I run wire especially to a shop I always step the wire size up at least one gauge. Obviously we don't know the history of your shop or wiring, but possibly someone in the past might have just installed larger breakers to get more power without upgrading the wire. just something else to check. Also check the incoming wiring from the power company. If your living in an older house you may have older smaller wiring coming into your service panel.

whatever you do, you need to get this resolved that surging is not good on the transformer in your welder. I can imagine you probably spent big bucks for such a nice welder, I would hate for you to only get a couple years of use out of it due to "dirty" power.