just thought i would throw my .02 in and say that one of the major things that causes the speedo to bounce is the bushing in the head being worn out
the origional speedo cables were slightly too long and this put excess pressure on the bushing, causing it to wear out, which makes the needle wobble a little bit and hit on the cable causing it to bounce. best thing to do for this is either have somebody rebuild the head, or replace it. unfortunately for me, since my car is a ssv, its really hard to find a 160mph speedo, so i had mine rebuilt, works great now though. good luck.
Ryan