Your starting point with this kind of problem should be change the needles.Once you have fresh needles in your machine check the machine is correctly threaded & you have no fluff collected between the tension disc's.If this fails to eliminate the problem you should check the point of the left hand looper situated under the needles ensuring the point is not rough.If you find you have a rough point gently remove the burrs using emery cloth.Set all your tension units to between 3 & 4 and set the first 6 or so stiches turning the machine by hand.
Wyn
August 2010