Why does my John Deere 762B scraper keep dropping out of gear?

[Elizabeth Williams]

I'm having an intermittent transmission issue on my late-1980s John Deere 762B elevating scraper. It shifts into gear, but then immediately drops out and the display flashes 0. If I unplug the wiring harness on top of the transmission, the display flashes 4 instead. Has anyone seen this before?