Speedo still not working!

[1986Silverado]

Got the speedo from a board member Friday and installed it Friday....well it is still not working right. Cable is new. It took it a few seconds to start moving, when it did it was still bouncing and clicking like my old one. So I got it home disconnected the cable from the tranny and speedo. Pulled the cable from the cable housing enough to put in the back of the speedo, then snapped the housing onto the speedo, reinstalled cable back into tranny. Well the bouncing quit but now it takes a few seconds for it to move and then it instantly jumps up to about 25mph the goes up to about 40mph and stops moving The cable is not twisted anywhere and does not seem to be binding.............So what could be wrong now?