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O/D is tempurature sensitive is the coolant temp remaining normal? Are there any engine codes for the transmission?
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if the coolant is below normal this will explain why the O/D is kicking in and out. The thermostat is sticking open. Change the thermostat and refill and circulate the coolant and re drive with a normal operating temp on the gauge and see if O/D works then. It is very Temp sensitive and will not allow O/D below a set temp. The codes can be read by jumping TE1 and E1 in the check connector box by the master cylinder and reading the flashes of the check engine light but if the engine is running to cold this is more then likely the problem.