A few weeks ago I was half way through a 2 hour drive to our vacation campground when, after getting caught in stop and go traffic on the interstate, my car dropped out of fourth gear into third while I was going 65 mph. It's terrifying to see the tachometer suddenly jump to over 4000 rpms. I took the first exit and began making my way on the side roads. I found that by mashing the accelerator and then letting off, I could get it to shift back into fourth gear, but if I ever let off the throttle most or all the way, it shifts back down to third gear.

After driving it a few more times, I found that when the car is driven when it's cold and if I don't stop for more than a minute or two, the car shifts into fourth gear properly. If the car is warm and it's sat for more than a few minutes, it won't shift into fourth without excessive coaxing (mashing the accelerator and letting off, then keeping the pedal in a fixed position).

I changed the transmission fluid and disconnected the kickdown switch to no avail. I inspected the vacuum lines and I don't see anything out of place, but I easily could have missed something. Is the transmission really on its way out or could there be some easier fix?

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1987 300SDL