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My old Pfaff used to play up sometimes like this & usually it was the tension discs needing adjusting only very slightly. On one occassion I discovered it was due to an uneven threaded bobbin. Sorry other than that I don't know what to suggest - maybe:
Stroke it gently & tell it Mummys loves you - SO GET TO WORK :lol:
Tisha - Good Luck :wink: