Speedo Help

[KABO5e]

So here's the story. When I drive my 240, the speedometer works half of the time. If it works, it works fine. What I feel is 20 shows as 20. When it doesn't work it shows 1/10 of what I feel I'm doing, So 20 mph shows as 2, 30 shows 3, or needle BARELY moves off of 0. Higher speeds get farther from 0, stop brings it back. After some Google-ing, I thought maybe my speed sensor was bad. I pulled another one out of my box-o-parts, put the shaft on a drill, spun it and got voltage on my meter so I figured that this speed sensor was good. I put the car up, swapped the sensors, dropped it down and took it for a spin. My speedometer worked. I parked my car and went to bed. Next morning, I go for another spin, Speedometer doesn't work(doing the 1/10th thing again). So now I'm thinking that I've got a short somewhere. I put it back on blocks, and plug the first speed sensor into the plug near the transmission and spin that one on a drill while using a meter on the plug that goes into the cluster and get voltage readings there. I plug the black plug into the cluster and the speedo gives me readings from the drill. Now I'm think looses connections. I jostle all wire connections while drill is spinning and cannot get the needle to drop without lowering the speed of the drill. I take apart, spray with electronic cleaner, clean and make sure that all plugs are put back tight. Still getting readings from drill. I plug the speed sensor in the transmission back in and drop the car. Speedo works again. Next morning it doesn't. I am now believing that my speedo may be the issue here. Any one have any ideas on test I could do to confirm this? All other gauges and lights have been working without issue through all of this.