After cleaning and oiling my new Pfaff 30 sewing machine, I noticed the stitch length lever jogged back and forth a little while the machine was running. I could hold the lever in one spot but it would slow the machine down. For a while I couldn't figure out what was causing this strange behavior but determined it must be something to do with the feed dog fork.
Once I pulled the fork, I knew it was the cause. The small square sliding block that fits into the stitch length lever (little square thing in the photo above) was tight on the feed dog fork stud. Thinking that it was just dirty or gummed with old oil, I cleaned it all in acetone, but to no avail. The sliding block was still tight.
As a last resort, I used some 600 grit wet/dry sandpaper and gently honed the inside bore of the sliding block just enough so it was snug on the stud.
Now the sewing machine purrs along nicely without the stitch length lever bobbing back and forth. The only thing I can figure is the sliding block has always been tight, since new. I had no other explanation for this, unless someone had been fiddling inside the machine at some point in its life and replaced one or more parts that didn't fit well together. I don't know.