Possible Transmission Issues, 88 Mercury Grand Marquis

Yeah it's an old car, so of course if there are transmissions issues that shouldn't be a surprise due to its age.
Anyway.
It has about 150,000 miles on it, the behavior I've noticed is this.
It has just started "roaring" when it shifts into overdrive. Also, it's reluctant to downshift at lower speeds even if I floor it. It appears I would have to practically make it do so by using the "2-1" values after "D"
It has never done this before, it just started. I did add 1 quart of transmission fluid (it was a tad low), but so far--about 3-4 miles after having added it--the behavior remains.
Also, earlier on--BEFORE these behaviors cropped up--it sometimes would roll downhill when in PARK because park wouldn't engage right away. Repeated efforts in any given situation would cause it to engage, at which time all was fine for that period. Lately, PARK works perfectly.
It upshifts fine as it goes through the gears, although during takeoff if I hold down the pedal to try & make it STAY in gear longer it seems to want to go ahead & shift early anyway. It does downshift normally when I slowdown to make a turn, and then upshifts normally from there.
Is this a sign of impending transmission failure? Would a good idea be to go to a transmission speciality shop and have them look at it to determine any causes? If they uncover anything, would a replacement or rebuidling of the transmission be likely? Also, (sorry if this is too many questions at once) what would be a reasonable price for them to charge just to look at it to see what's up?
Tips?
LRH
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It was nothing more than the downshift (or "butterfly") cable had become disconnected from the throttle control thingie. A mechanic reconnected it for $20. Fortunately the transmissions didn't burn out on the way there (about 8-10 miles). I should've known it was something like that, I noticed the accelerator had a MUCH lighter feel to it.
Problem fixed. Shifting is totally back to normal.

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"Larry R Harrison Jr" wrote

I assume he replaced the nylon bushing the cable's "pin" goes through at the throttle body lever? It's a well-known problem part that disintegrates over time and causes many AOD deaths.
How? It's not a "downshift" cable like the old transmissions used. This one directly controls the shift points *and* tranny fluid pressures to the bands and clutches. When the cable falls off, it basically tells the transmission that the engine is at idle, which drops the internal oil pressures way down. When you step on the gas to accelerate, there's not enough pressure to clamp the clutches, and they slip like mad until they burn themselves out.
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So, I assume it's safe from "burn-out" now that the clamps ARE getting pressure? That's what the mechanic told me--that if it had "burned out" prior to his replacing the cable, then it was too late but if it totally resumed normal shifting (which it did) then it was fixed and that I escaped "burn out." Sound right? Or, should I maybe change the transmission filter/fluid ($75) as an extra precaution?
LRH
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Larry R Harrison Jr opined in wrote >

Hmm... if it WAS slipping, that would sound reasonable to me.
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