Arms jerking and stopping when going to trace

Hi,
I have searched the forum and was actually going to sell this machine which I LOVE but just don't use it enough. I have it all set up to do some practice on and lines up cleanly, well oiled etc. everything goes well until I go to trace and the arms jerk with frame to side and then stop and error bell comes on. I have checked everything and turned off and back on and then called repair guy but he is out of country.
Any ideas on what is going on? It seems like it is something simple and dealt with this before but we are in the middle of packing and moving and my brain won't work!! HELP!!