Speedo Shift Problems When Wet Outside

I run a 1996 E-350 Diesel van. When it rains or snows outside, for about 2 or 3 days after, the speedo hits 25-30 MPH and then drops to 0 and the engine runs through. I had the Speed Sensor Replaced about 2 months ago and seems to shift fine when dry out. Where might the problem lie?

The speedo is signaled from a sensor on the rear axle. The wires run up the LEFT inner frame rail to the connector undre the brake master cylinder. I suspect there are bare wires or a loose connector pin in one of the three connectors between the sensor and the connector under the hood. It could even be AT the sensor itself.