308 Speedo works intermitently?

Its telling you to use the search function to help you figure out the fix

 

I had a similar problem in my mondial qv, it turned out to be a faulty sensor in the gearbox, replaced it and no problems

 

If its not the sensor, send it in to Pal Alto for a rebuild. Known problem. Big PIA to R and R

 

But don't make my mistake and after testing one thing assume it's the speedo. I blew a $75 bench test fee + a chunk to FedX it coast to coast for nothing. Oh yeah + a new sensor that's sitting on a shelf as a spare I guess. My case turned out to be a simple connectivity issue - the engine wiring harness connection was loose. How 'bout that

 

I had the exact same thing happen on my 308QV - turned out to be a bad speed sending unit. It's a very simple replacement - just jack up the back end - crawl under... it's easy to spot. The only thing you'll need is some sort of a tool to remove and replace the new one w/o crimping the wire. I made my own out of a 32mm socket (you can find easily at AutoZone or PB) - I simply used a grinder to create a slot for the wires.

I've included pics of the sensor and socket... good luck! Oh - one more thing,... don't use the numbers on the sensor I'm showing... it's actually for a testarossa (note it has a different pig-tail connector, I ended up having to splice my old one back on!)
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That's where I'd start!