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My name is Dale.
More than likely the filter in the transmission is partially plugged up and when the fluid is old and thick the fluid cannot get through the filter to apply the clutches in the transmission.
The solution is to change the filter and flush ALL the old transmission fluid out of the transmission. 90% of the time this will cure the problem.
Beyond that (If it still gives you problems) you need to take it to a good transmission shop and have them run the pressure tests on the transmission.