Check the OD lockout on the transmission. it's a mechanical sensor that prevents the OD from engaging in any gear but 4th. the litle rod will wear down until it is intermittent, and wil do strange things like engage on hot days (metal expansion) than on cold days (metal contraction). It will not work for no reason at all, then work just fine. It's easy to pull, and you can test the system by pressing the rod in with the car off, but ignition on (not running). Or, if yours has a washer on it, remove the washer and put a little sealer on the threads, and put it back in place. the removed washer will give the sensor enough depth to work properly.