Speedometer Issue
 

I have a 1996 Explorer 4x4, automatic transmission. The speedometer started acting erratically and would quit working. I replaced the VSS in the transmission tail shaft but it did not take care of the problem. I have bench tested the sensor and it passes. I put a drill on the sensor while it was hooked up to the vehicle and was able to hold the speedometer steady at 60 mph for two minutes but when it is installed in the vehicle it still bounces and quits especially when driving aggressively. I checked the plastic gear inside the transmission and it seems fine but I can not find a replacement for it to check it. What other options are there and where would I find a replacement? Thanks for any help provided.