I have this machine, and about 6 months ago it got a mind of it's own. Would stitch fine for awhile, and then start sewing stitches 1/2 inch long. Or zigzag fine for awhile, and suddenly switch to a straight stitch. Took it too my local repairman, and he found a damaged part, replaced it, and thought he fixed the problem.
Nope, not fixed. Refunded my money.
I called the next Pfaff dealer, a good 65 miles away. That repairman told me to try changing the light bulb. (It was not burned out.) He said he doesn't understand why, but changing the light bulb seems to fix some of the stranger problems with this machine.
Did that, and six months later the machine is running fine. Still scratch my head when I think about this.
Don't have a question for you. Just wanted to share my head scratching experience!