My son drives a 1983 240 GL with under 125000 miles on it. Recently, he had an oil change, and I suggested he run a container of fuel injector cleaner through because he had said his fuel economy seemed to be down some, and it was idling rough.

On the tank that had the cleaner, he was driving on the highway, and as he drove the car developed a propensity to downshift inexplicably, especially much sooner than normal when going up hills, and sometimes for no reason at all. Over time, that subsided (this was about 2-3 weeks ago) until the last couple of days. He now tells me he had a slight recurrence of this problem starting a couple of days ago, but he just called because he has been on the highway for an extended time and the car is once again downshifting a lot. As an example, he indicated as he approaches highway speed of say 65 mph, the car downshifts to 3rd gear, so he is suddenly turning 4500 rpm instead of, say, 2200. So he slows down to save gas, but that's pretty frustrating.

Anyway, we last tuned the car up about a year ago (plugs, distr cap, spark plug wires). I am assuming the problem is either some kind of sensor (but I don't know what), perhaps the oxygen sensor, or something to do with the fuel system; or for some reason something that controls the automatic transmission believes a downshift is necessary when it shouldn't be.

Any diagnostic experts out there with an opinion? Thanks.
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83 240 GL