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The tcc controls the converter lockup not Overdrive. By the symptoms that you are giving me I would lean on the Manual Lever Position Sensor. It is mounted on the top of the transmission right where the shift linkage is connected to the transmission.
We do have a problem with that sensor commanding a downshift while driving at freeway speeds. I even had one shift the trans into 2nd gear while at 70 mph.
When you say you are at cruising speeds, how fast are you traveling. Speeds around 40 to 50 mph and the od will come in and out on a regular basis, depending on load. (throttle setting)
That is the tcc or the converter clutch coming out. 200 rpms is how much of an effect that the converter clutch has on rpm. Are there any trouble codes?
We do not have many problems with the solenoids for the tcc. but we do have problems with the converter clutch and with the valves in the valve body.
That DTC is for Excessive slip detected when tcc commanded at 100%
That is always a converter failure. You may get away with a trans flush, but I doubt it. Every time that I have that DTC, I end up replacing the converter.