Mig welder problem

[Yondering]

Thanks guys, that's a lot of great advice. Dedave, lots of stuff there I didn't know so that'll be helpful when I get a chance to dig into this further. (Probably the next time I need it - ha!)

The description of liner issues does sound like a likely culprit. I used the welder briefly the other day and cranked down on the roller tension more than I've needed to in the past, and that did help a bit, so it may well be a liner issue.

[scottly]

This doesn't sound like a liner or feed issue to me. I have a 25 year old Lincoln SP 135 that has seen lots of use and countless spools of wire, both flux core and solid, and it still has the original liner. I did have a failure once of the circuit board that drives the feed motor, and the factory tech support guy I spoke with told me the board had been re-designed, and replacements wouldn't be available for several months. I told him we couldn't wait that long, and if I had to buy a new welder to keep production going, I would be thinking twice before buying another Lincoln, but if he could fax me a schematic of the board and I could repair it, I would be happy. He faxed me the schematic, and it was fixed with $5 worth of Radio Shack parts.
Your problem sounds like a bad connection to the torch; check the lug at the end of the lead used to switch polarity where it connects to the block where the torch plugs in. My welder had similar symptoms to yours, and I found evidence of arcing and eroding of the bolt. I also recall there was an issue with the screw that retains the torch to the machine?

[Yondering]

That sounds like it's worth checking, thanks!

[Willy]

Yup another good point worth checking, either way it sounds like the machine is not delivering full amperage at the weld. Another point of contact to check is the connection between the gun and the cable all the way to the contact tip itself. You'll have to disassemble the gun to check these items but it is a simple process detailed in your owners manual. As long as all connections are clean and secure you'll be good to go.
Lots of low hanging fruit to pick before going deeper.