68 speedo

I would bet that either you have a broken speedo inner cable or the speedometer itself has internal problems. There are several instrument repair people around, some even post on this board. Try searching the forum for speedometer and you'll probably find some.

A google search for "speedometer repair kit" turned up several hits, not all were relevant to mopars. The best one I saw was this:
http://www.ebay.com/itm/Speedometer...Parts_Accessories&hash=item5aeb45e629&vxp=mtr

That item is just the inner cable that you cut to length. The seller also has stock NOS speedo cables.

If you take the gauge out of the cluster, you can turn the connector on the back by hand and see if the meter has internal problems.

ATB

BC