First rule with any troublesome autobox is to do a fluid change - if it's the cause of the problem it'll cure it and if it's not the cause it'll eliminate it as a possibility.
Carefully research the autobox fluid recommended for your car and then find out if the latest version of the fluid is backward-compatible (most are) and then use the latest version of the fluid - eg, if your car needed Dexron 3 originally, use Dexron 6 for the fluid change.
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