January 24, 2016: I bypassed the wiring harness for the correct sensor this time, and was able to complete a print that would never complete before. When I was determining where all the wires went that I would have to bypass, I also measured the resistance between the sensor and the controller board connector. Three of the four wires were about 1 ohm. One was 5 ohms. I suspect it has a marginal connection and fast movement shakes it loose. Now I need to figure out how to fix it. The wiring loom is very tight.