The speedometer in my truck works every now and then. Sometimes it does, and then sometimes it doesn't. Any solutions?

There is a plastic drive gear inside the trans. Follow the cable down to the trans and remove it, you will find the gear. It is probably missing a tooth or two. Replace the gear if this is the case, and if not it is a broken cable or a faulty speedo head.[the part in the dash]

My guess is that the speedometer itself needs replacing. Speedometers on old Chevy cars and trucks were mechanical. A rotating magnet played off against a spring-restrained cup and needle. It's a balancing act that eventually stops. The contraption looks like it was designed by school kids, not engineers. Go to a junkyard and try buying a replacement speedometer. There are no guarantees that even it will work well. I've done it before and it's not much fun. It's amazing how primitive old speedometers were and how cheaply dash components were made. Tear it apart and you will see what I mean.

Good advice given above.
Follow the speedo cable to the tailshaft on the transmission
Remove end of cable or gear which ever set up it is. The gear is either
Of plastic or nylon construction. Look for worn or missing teeth.
If that is not the problem check the cable {new one goes for about $ 20}
I would bet it is more likely one of these 2 parts are bad before the speedo head.But if you have to pull the head it is VERY easy on a 70's Chevy truck.
And finding a replacement at a parts yard will be no problem as there a
Tons of them out there.