So I got under there and took the plug out of the diff (was safety-wired!) and cut the wire up inside the storage compartment, and then replaced that section of wire (I voted it "Most Likely to Have A Break In It") with some test-lead wire.

At first I thought I had it, the speedo worked, but then it didn't, and it started acting the same old way (jumping around, then suddenly working after a hard turn or hard brake, or just in the middle of a drive with no real provocation).

So now what?

I already tightened the three connectors at the back of the speedo, and have reached up behind there at speed (not easy) and tried to affect it with no success.

I find it hard to believe the the run of wire from the back of the car to the front could have a break, since it's in a protected tube with lots of other wires that aren't broken.

The only things I can think of are:

1) Tighten the connectors on the diff plug, which I didn't do because I was afraid of breaking that little grey plug-case in the cold.

2) Solder the connections onto the back of the speedo.

3) Find a ground somewhere that only affects the speedo and nothing else in the gauge area, and fix it.

4) Replace speedo ass'y.

Anything else?

5) Get another car, is rising with a bullet right now....&^)

Thanks, as always.