86 pontiac 6000 speedometer goes crazy with on/off overdrive

hi,
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
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