Speedometer problems
 

'81 longbed here, mostly stock. Ever since I sandblasted too close to the gauge cluster, I've had a speedometer that liked to stick at random speeds and random times. I once got pulled over for 35 in a 30; my speedometer was stuck at 24 the whole time, including as I showed it to the officer. About 2/3 of the time it worked normally.

I've cleaned it several times since it started. Last time it was a pretty thorough affair with lots of Q-tips, PB Blaster, compressed air, and attention. It still sticks a little but seems to have improved. There's a weird new problem, though. When cold, the speedometer will read high by about 50%, or more, at different speeds - going about 25, showing 40; going about 35, showing 50. It's definitely not sticking. After a while it suddenly goes back to reading normally.

Seeing how it works, I can't imagine what could cause this. I don't think I bent or broke anything inside...

You've probably got a little piece of grit stuck in there. I'd find a new (used, of course) one and very carefully swap it into my cluster, if I had your problem. Also, a very thorough lubing, with a good penetrating oil, might help.