Speedometer works then doesn't

 

It's electronic. Probably a bad connection somewhere.

 

Could also be a bad speed sensor down on the tranny.

 

theres a speed sensor pluged into the very rear overdrive housing. those wires that lead to the plug are well known for going bad and exposing wires. plus that connector also gets worn over time & breaks. so the 2 combined exposed wires and the broken connector usually caused the sensor to malfunction intermediatly. they sell a harness and plug that you can sodder and shrink wrap to old wires where it went bad. $15 part if you DIY. GOOD LUCK