86 pontiac 6000 speedometer goes crazy with on/off overdrive

I have a pontiac 6000 LE and i notice that on certain times of the day my speedometer goes crazy and at the same time, the car seems to go in
and out of overdrive by itself, it wont maintain rpms when the spedometer goes nuts.
eventually the speedometer stops working completely and then there is no more in and out of overdrive problem.
any reasons why these things may happen?
thanks greatly
Posted using the http://www.autoforumz.com interface, at author's request
Articles individually checked for conformance to usenet standards
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.