Thank you sdwally - thinking about the problem some more I wonder if it's because the welding machines have the auto-idle feature, where they stay at idle RPM until you strike an arc, then they rev up. In the course of a day's "burning", that adds up to a lot of lo/hi RPM cycles. Plus, the cycle is from idle to full throttle every time. One could leave the engine on the "rabbit" setting constantly, but I think that would likely create more problems.
I'm hoping the older style tube/cap will interchange - as I mentioned, 6+ years without a problem with that style.