91 cv 5.0 302. someone please help!

Hey, my name is jay. hopefully you can help me! If you dnt mind. I have a 91 crownv. 5.0 302. The tranny shifts hard into overdrive. Other gears seem to be smooth. Down shifts in all gears are good also, even out of OD. Ive tried adjusting the throttle valve cable. Helped a little, but still not rite. My speedometer is off on the mph by 3mph+. Less off the slower im going. Up to nearly 8mph when at 55mph. Give or takea couple mph. This is the second tranny. 1st tranny did the same EXACT thing. Could my speedometer be causing this? Or maybe confusing my computer? If i SLIGHTLY let off gas then QUICKLY but slightly press the gas i can make it shift alot better. Oh yea, if excellorate at a high pace it will switch better but thats probably because im at a high rpm and speed. Somewhere around 65+. There isnt any type of slippage in any gear. Also once im in overdrive, if i come up on a slight upgrade in terrain the car starts losing speed, so i slightly exelorate and instead of trying to slowly pick up speed the tranny quickly drops out of OD and takes off. Seems to be sensitive when pressing gas to gain little speed thats actually needed. By any chance do you know wats going on? Oh, and my exhaust needs to be replaced, but that was ok with the first tranny and a few months with this one, so im guessing thats not te promlem.. THANKS FOR ANY ADVISE U HAVE TO OFFER!