speedometer slows down, then stops working?

Detroit80

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Truck is a 1990 CCLB, 7.3 w/Banks, 5 speed trans, 4WD. Bought it used a few months ago, and have been slowing working through the numerous issues.

Current problem - Speedometer will start slowing down after a few minutes, then quit altogether. Such as I'll get on the freeway, get up to 65mph, and the speedo will read 65mph for a few miles. Then without the truck slowing down, the speedo will drop to reading 60....50....45...then finally drop to zero. Odometer seems to stop at this point as well.

Oddly, it seems to work just fine for a few miles again after shutting the truck down and restarting it, though I haven't yet tested if this is an immediate thing or only after letting the truck sit for a while. Sometimes it will be fine for 50+ miles, other times it will stop working after a mile or two. Seems to shut down quicker the faster I go.

As I normally have a GPS with me anyways, I wouldn't care so much about the lack of speed readout, but my next project will be to fix the cruise control, and I suspect these may be related.
 

no mufflers

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I would check the speedo cable and drive gear at the transmission and go from there. I doubt it's the cluster but those are the easiest places to start.
 

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