HELP!! 91SL transmission trouble

[Marc-1]

Gilly, if you're out there I need your help again
Here are the simptoms; The transmission has not been up shifting at the correct time. Sometimes it downshifts when it shouldn't and when needed, (starting out by pulling out in trafic) will not down-shift correctly. Didn't let the car warm-up this morning and about 6 miles down the road I heard a big bang and the shift lever rattled loudly. At the same time the service engine light came on. I had noticed this problem once before (not as severe) while traveling down the highway a bang then rattling of the shift lever. It went away the first time and this time the BANG was very loud and the whole car shuddered. Is it possible that the relay controling the shifting is engaging two gears at once? It seems to be a control problem since the transmission still works fine, outside of the correct shifting problem. I'm afraid that there may be some severe internal damage now and hate the thought of driving the car to the dealer. But before I make an appointment for diagnostics or service I wanted your thought on this "control module". Thanks