My Speedometer is Malfunctioning?

I have a '97 Civic with 129,000 miles on it. My Speedometer suddenly jumps to 20 mph when I turn the key to on, but haven't ignited the engine. It doesn't work properly but half of the time. The needle bounces around, then starts working properly, then starts bouncing again. My odometer stopped working, too. I replaced the gauge (it includes both speed and odo). This cured the malfunctioning odometer, but the speedo is still not working. So, I got a new Vehicle Speed Sensor, put it in, and the needle STILL jumps to twenty and bounces around, then works properly, then starts acting goofy again. This is driving me insane! Please help?