I have recently replaced the speedometer cable in my 67 after the speedo was jumping around and not working properly, however i still have the same issue after replacing. where would be the next place to look for the problem?
Next part I would replace would be the actual cable. It can be removed and replaced inside the original housing (or you can buy a new cable in a new housing from the reproduction houses). You use the old cable to verify the length of what your replacement should be. Then when sliding the new cable back into the housing use lots of dry graphite lubricant to keep it spinning freely.
It wouldn't hurt to check the gears as well (one screw removes the retainer). You don't normally tear up the gear until the cable stops spinning all together. Yours is just in the sticking stage right now. If they are worn you can replace them with equivalent aluminum gears that were used in all light trucks instead of the plastic gears used in cars.
I use this company for all of my speedometer parts: