This was really weird.  Cutting a small piece with lot of compound curves,etc.  Making multiple .100 (inches) deep cuts to keep from hogging the small Taig mill.  First couple of passes were perfect.  Then, suddenly, the end mill turns at a right angle to the programmed path.  The dros were indicating the table had moved a lot further than it actually had. Checked the code and counldn't find any obvious errors.  Re-ran the program.  Was doing fine then - bang - mill suddenly (at a different point in the code) reversed itself and started cutting a bad angle -  Same routine, re-run, its okay for a while then #%*$@@!  Piece was trashed -

When make a new part I generally test new pieces by running in wood rather than aluminum - cheaper.  Noticed that everytime the milll went haywire, I was using a small shop vac to clean away the sawdust so I could see what the machine was doing.  It appears the motor on the shop vac was producing a signal what was picked up by the mill.  It coulld be that the drive motors themselves have a short (but this had not happened before), or the vac has a short.  It was plugged into the same circuit the computer and mill were on.  The vac was at least 30 inches away from the closest point of the mill.   

Finished running the part (without turning on the vac) and no more errant lines.  I will have to run more tests to see if, in fact, the vac was causing the problem' Or, see if there is another cause.

Bob @ BobsShop.  - Scratchng his head and watching the mill.

Shop-vacs or any sort of dust collection will produce A LOT of static electricity.  I would guess that is what was causing you troubles. 

Bob, this is pretty common with routers most like myself have added additional ground rods and grounding wires between the router tables, spindles, dust collectors and anything else that may generate an electrical field.
If you don't want to run a ground wire on your shopvac, switching your hose out to an explosion proof hose will more than likely take care of your problem. Its a canvas dust hose I believe made for the grain elevator industry, its about $2 a foot for 4" at our local industrial rubber belt and hose supplier.