Is it a reproduction speedo cable?
I put a repro in, and there is supposed to be a little crimped on ferrul at the speedo-end of the cable, that keeps the cable in the sheath, rather than slipping down into the trans.
It's possible yours is slipping out of the end of the speedo head and just catching it erratically.
Your speedo could also be about to die. Mine did that right before it croaked.
I ended up finding an original speedo cable set-up, washed it in the parts washer, and lubed the cable with white lithium grease. It's going back in the next two days for a 1,000 road trip to Missouri. It'll be nice to know what I'm getting for gas mileage, and how fast I'm going!
-Brad
