Re: Another \"Whats this\" question!!
Like everyone else has said already, its the "AC CruiseMaster" cruise unit. Another thing to try to make sure it is indeed that part, is to unhook your cable at the case, then hook up a cordless drill to the cable itself, run in reverse mode, and watch the speedo-does it move, and how well? If it runs at a steady 10-15mph or so (about max speed for my 18v cordless), chances are your cruise unit is good-if this is the case, go back to the adapter going into the case and replace that little "key" piece that connects the driven gear in the case to the actual speedo drive gear, making your speedo turn. Those tend to fail more often than the actual cruise unit.
However, the units themselves are prone to a magnetic problem which will cause them to seize entirely-if that's the case, and with the drill, you have no movement at all on the speedo, replace this. It's about $50 or so if I remember right from your local parts store-somewhere in that neighborhood anyway.
If your problem is more of a bouncing speedo while driving, maybe rests at a given speed, then will suddenly jump and bounce around, that's the actual "key" itself down by the case. Again, they fail quite often, but luckily are cheap and easy to replace. Another thing to try, even though you have already replaced the cables, is to lube them with a good graphite. If you can find the old style can that fills with graphite, runs off your air compressor and a drill-those work best. Basically the air blows graphite up through the can into the speedo cable line, as you run the drill, thus spinning the cable and drawing the graphite up to the other end. They work great, but are hard to find anymore. Even new cables occassionally need a little lube work.
Hope this helps, and Best of Luck,
Mike