Speedometer Question/Problem

[Nodnarb76]

Hello, I have an 87 Suburban 2wd 3/4 ton with the TH400 transmission.
When I got it, the speedometer never worked. I heard the cable was a common problem so I replaced it, and still did not work. I replaced the driven gear next and it worked for about 8 miles then quit. I can spin the transmission end of the cable with a drill and the speedomter moves but still doesn't work when I reconnect it to the transmission. On some cooler/humid mornings I have noticed the speedometer works for the first couple miles then stops.

Any ideas as to what is going on?