Bernina 170e Top thread catching around bobbin and...

Question

My 170e will embroider fine and all of a sudden the top thread bunches around bobbin casing and knots up on fabric and in top plate needle hole. Nothing specific seems to cause it. Just randomly happens. Sure can put an end to a nice project. Also if the machine is fixable any ideas where I can find the older software? I have Artlink v3 and CPS v4 and have to run on windows 98. I wondered about Artista. I am limited on putting together designs and especially fonts.