Drat - my machine is down

I have been having trouble with my sewing machine for about 4 months now - it's been barfing thread all over the back of my blocks......so just before Christmas I took it in to be serviced, and when I got it back I thought it was better......nope, it's right back to where it was and acting up even more!  BOOOOOOO!!!!!!  So now it's back in the shop again.  I can't figure it out.  It'll sew along quite nicely for a while and then just start misbehaving and from the top it looks fine, but the back is all loose and loopy, then it'll smarten up and behave again.  Doesn't seem to follow any pattern (pun intended ;>P) just comes and goes at will.....Lucky that I have my old machine as a back up while the new one's in hospital - might have gone into withdrawal otherwise!