This probably wont help you...but if it makes you feel any better I have a 1994 Ford Ranger 4X4 and it does the same thing. I dont know why it does it or how to fix it. But I live out here near Kansas City, MO and there is a specialty place that is actually called KC Speedometers (Catchy huh?)
Jumping in the speedometer is most commonly caused by either (a) a gummed up speedo cable housing, or (b) a kink in the speedometer cable housing causing the cable to bind in the casing -or- (c) heat damaged casings have alloowed molten plastic to enter the housing and bind it up like Cheeze & peanut butter fiend. Also, (d) the speedometer gears may be damaged from someone replacing the cable gear with a gear incompatible with the tranny. I say incompatible because the cable gear may be correct, but the gear within the transmission may have been replaced at some juncture with the wrong one. Ford had left and right cut gears depending on whether the cable went in on the left or right side. Often, when the tranny is rebuilt, the tech doing the work reaches into the bin of gears and throws one on not noting whether it's correct or not. The transmission is right, just not that 2 1/2 inch diameter gear. When you first insert the wrong cut gear it may work briefly and then become chaotic over time. by that time both gears will have had their teeth sufficiently smushed to render both next to useless.
Tom Kubler, Long-time Mustang Enthusiast & San Antonio Mustang Club Founder
Mine did the same thing. I pulled the inner speedo cable from the sleeve, cleaned it up with solvent, lubed it real good with light oil and reinstalled it. Problem went away. It's easy to get that inner cable out, just disconnect at the speedometer and it just pulls right out of the sleeve. Good luck
I can say from experience that a '67 C4 and a '73 top loader have opposite pitch trans speedo gears. When I did the C4 to 4 spd conversion many years ago I reused the speedo gear on from the cable that went to the C4. It seemed to read the correct MPH and mileage until the needle started to waver - then it quit. Yup. chewed up the speedo gear - oops.
Hopefully it's like the others say and you just need to relube your speedo cable. I got a tube of the stuff from Pep Boys but any auto parts place should have it.