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Also have a good, good look at the shift control/kickdown cable. Before you drive or start the car on a cold morning, turn the throttle 'spool' (where the throttle and shift cables attach at the throttle body) all the way open. If the cable doesn't return immediately when you let the spool close, there is a problem. If it is binding (always cold mornings are the worst) it will do EXACTLY that. Hilly areas exacerbate the problem since one has to step on the gas harder to get rolling and the cable therefore gets pulled out more, but can't retract. It will hold the gear until it either warms up and functions properly, or the trans gets to the maximum and allows it to shift regardless of the shift control cable position.

However, if the kickdown cable is functioning properly (should be virtually no resistance on it at any given time), then I suspect that your problem is valve body related. Sometimes a trans flush helps, but sometimes a used trans might be a more effective solution.