I am having the exact same issue. soon as i turn the key to accessory power, the needle jumps to 20 mph. Then at low speeds, it wobbles back and forth, past 20 mph it stays still. Very rarely, it will randomly work mid drive, showing correct speed, then "glitch" and go back to 20. Today I unplugged the VSS connector, blew on it, wiggled the cable a bit. Turned the car on and it went to normal! drove around the block, worked entire time. Then when I came back out later today to start it again, back to same issue. Cruise control works, and I know it can get a speed signal, so I am guessing the VSS is in working order? Anywhere I should focus my attention to? I am glad I didn't buy a replacement gauge cluster because the speedometer will work randomly and I don't believe the problem is there.

 

Post car information.

 

Could either by the vss is on its way out or the soldering points on the back of your cluster need resoldered. iVe had a bad vss and bad cluster before, bad vss usually is a jumpy needle, there is a test to check the voltage coming from the vss. I believe RONJ says it best, dont guess trouble shoot.

 

I had a similar issue except mine didn't jump to 20 with the key on. I had a spare cluster so I swapped it. Problem solved.