Hi! if no signs of transmission slipping, and fluid condition does not show any signs of metal or burned, only possible cause for this downshift can be internal in the valve body.
If up to it, I recommend removing the pan, and valve body (all 8mm bolts). Next remove the separator plate cover, (flip over on table, and remove all 10mm bolts) be careful not to lose any of the eight check balls and one fluid restricter. Next, drain valve body and push all valves against their springs with a pick, if any seem to be stuck, you can try to remove it and clean it up carefully by moving the valve in and out of the bore rapidly.
An easier repair would be just to replace the valve body with a remanufactured unit.
Thanks so much, and I hope this information was helpful. Do not worry about any electrical solenoids being at fault.
Okay, great question. I have on my own vehicles, drained about one quart, then put
four transmission conditioner additives in, (Dont flush, just install about a quart of Lucas trans additive) drove it for about a week, then flushed transmisison, and the problem (simililar to yours) went away for good. Definetly worth a try, and much less expensive then what I recommended above. Now, I can not gaurentee it will work with your particular situation.
Thanks so much for the update,
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