x axis shift during bed leveling

Hi. I'm new to the forum. I recently completed a prusa i3 like custom build. The printer prints fine but I seem to have an oddity with the auto-bed leveling. I'm using the technique that the Lulzbot Mini uses with the nozzle and bed forming a switch. I'm even using their corner washers. When it works, it works fine. It does the orgin test fine but sometimes on the max x/min y or max x/max y the x axis will suddenly shift inward five or so mm. The tip drops and hits the washer on the first probing just fine. It seems to move between the initial probing and the second probing. I can't explain it and it doesn't occur every time. Of course when it does, the nozzle is now beyond the washer and I have to kill it quickly or it hits the bed.

I'm running the latest Marlin firmware. The hardware is Arduino and RAMPS 1.4. My belts are tight. Anyone see this behavior before? Any suggestions would be appreciated. Thanks.

John