Hi, i got an silverado 6,2 d -91, the speedometer works when it feel like it, sometimes without any problems and other times when i have driven a couple of miles, any sugestions what the problem is ?

 

That would more than likely be your speed sensor.
should be on the tranny or transfer.
It wouldn't hurt to take off the dash and check the conection on the back of your instrument cluster, and check the wires on the sensor.

 

just the speedo, or the whole gauge cluster? Had a '91 that had a bad groud in the dash, the whole thing would go out.