Trace on screen jumps all over the place, cant complete a shape

our arcdroid is a 2AM-ADP-220-1 .01-V2
If you go to trace a circle you can get so far round the shape then it will jump to an existing part of the trace that you have partially completed.
If you were to draw a circle within a circle for example if you got the outer one completely drawn and started on the inner the trace would draw a cut line to random placed between the two circles. The unit has previously been going well. we have checked earth bonding etc. Our plasma is a GYS-70 unit
Thanks Steve

You may have a snap setting on that should be off, in the upper left of the screen area