1997 Mercury Sable will not shift into overdrive

Car has 106k mileage. In winter it will not shift into overdrive until engine warms up, then it sometimes it will shift up and down a few times before it stabilizes. In summer it shifts fine when engine is cold, but (especially on a real hot day) after running for a couple of hours (I deliver mail, lot of stop and go) it will not shift into overdrive and sometimes downshifts hard and does not pick up quickly from dead start. This usually only lasts for a few minutes and then it works fine. Other than these instances the trans works fine and shifts very smoothly. Electronic problem? It seems like if there was a mechanical problem it would not shift properly at all. Help!