350 Turbo passing gear problem

Hi guys,
Working on an 84 Monte SS with a 350 Turbo tranny. When you kick it at 25 - 55mph it feels like it kicks down to the passing gear, the motor screams but it feels like little to no power. From a dead stop it's like a bat outta hell. Manually shifting from a stop (1, 2 D) works equally as well but when you romp it in drive, it feels like it's downshifting properly but there is very little increase in speed, there is no missing, cutting out etc.
Then sometimes it works perfectly, kicks down and goes like crazy
Car has the original motor and tranny and has appx 75K miles on it.
I'm pulling my hair out on this one and my kid needs the car to get back to college after the Holiday break.
Any suggestions would be greatly appreciated.

Thanks,
Unclegeorge