After extensive cleaning of the raceway the problem kept returning. I thought it was because the fabrics were shredding lots of threads but the machine would lock up after a short a amount of sewing.
I took the back of again to try and identify exactly where the stiffness was originating and notice what seemed like a lubrication point/grease nipple on the "big end" of the crank shaft. A few drops of oil there and the machine is now working like new again.
Marnie
June 2011