50 + year old plastic parts! I would be mighty hesitant about taking the speedo apart. Someone mentioned a possible kink in the speedo cable and I seem to remember that coming up on another group I frequent.
Also a warning that too much lube in the cable can result in it creeping up into the speedo mechanism, with bad results. It apparently doesn't take much gunk to REALLY mess up the speedo mechanism. I would first pull the inner cable out of the housing and inspect it for kinks and worn spots. It really seems to me that your symptoms indicate a cable, rather than a gauge problem
A google search for "auto speedometer repair" turned up several interesting results and some not so interesting. It might be worth looking into. Perhaps a couple of phone calls might get you some better opinions.
On 3/20/2015 9:07 AM, 'Paul L. http://1962to1965mopar.ornocar.org/ml-le' via The 1962 to 1965 Mopar Mail List Clubhouse wrote:
While I've got my instrument cluster out of the car I'd like to address a small issue I'm having with my speedo. While driving about every 5 seconds the needle will bounce a little. Usually about 5-10 mph. It will first drop down a few miles an hour and then bounce back up to the correct speed. I have had the speedo cable out and lubed a few years ago first with liquid graphite and then a second time with Vaseline. Neither one ever made any difference. I'm wondering if there is a problem with the speedo head and a way to lube it? I have an extra instrument cluster that I'm going to disassemble later today to get an idea if it's possible. Butwould appreciate any input from someone who may have done this before.--