Check the cable FIRST for to tight of a bend etc. I adjusted mine in one spot (right where it came through the firewall) and it cured it. I thought it was the speedo at first, but after chasing the cable I found I had bent it a bit too much in one spot. It would jump to around 50 and stop.
The way I actually found it was I drove the car a few feet and when it stuck at 50, I started wiggling the cable and it dropped to 0.
This may NOT be your issue, but it's worth a shot.