The following are quotes from the Bernina V6 Yahoo Groups:
This discussion was similar to what you describe.
"I have the 200/730 and I also had it stop in the middle of a large embroidery
design. I live 250 miles from my Bernina dealer, so I called the repairman
thyere and he told me that the speed needed to be calibrated, and he talked me
through the recalibration. It is not all that difficult to do. When I had the
upgrade to the 730, they gave me a print out that updated the manual to fit the
730. The procedure to recalibrate is in that print out. At that time, the print
out was available on the Bernina USA web site, and you could download it. I
haven't looked for it in awhile, so I don't know if it is still there or not.
Recalibration seemed to fix the runaway problem, my machine hasn't runaway since
I did it."
Regarding the manual supplement on
http://www.berninausa.com:
"It is still there, but if anyone goes to search for it, just type in
730. If you attempt to search using something like 730 Manual Supplement
(which is what it is) you'll get no hits! Just use 730 and then scroll
to the bottom and it's about 8 up from the bottom."
The manual supplement is a pdf so you can download it.