I think your model (the EH) is the electronically controlled trans. There's a computer or controller module that determines when it shifts. There's probably a variety of factors; vacuum, temp, speed sensors, etc. My guess is that may be the cause. Many older BMW's always seem to suffer from some electrical problems. I would carefully check all the connections, wiring, ground points, etc. and all the vacuum lines, coolant sensors, basically anything that signals the controller.
Don't be gormless; get some gorm!