1977 Oldsmobile cutlass trans problem

Hi, sorry if this post is a bit wordy. I got a 77 olds cutlass supreme brougham with a 350 trans and a 350 olds engine. It is all stock, and in fine working order. I spent $900 rebuilding the trans 3 years ago and have not had any problems. I had a shift kit put in along with replacing all the planetary gears and a new tci converter. Just recently, a problem developed that when you are accelerating at full throttle, like when you are going onto a highway on ramp. The trans does not shift into third gear, which happens at about 55 to 60 mph, until you let off the gas. Once it does shift, acceleration is strong, even past 70 mph. The car seems to bog down and slow down when it does not shift , but under any other conditions, it shifts fine and accelerates hard with no slipping from a stand still, or for passing. It shifts perfectly under "normal" acceleration through all the gears. Please help with any info you guys might have.

Kick-down cable [passing gear] in need of a little adjustment cable probably streched a bit ,also hope you have a trannie cooler if not you will cook your pump large o-ring external seal , the one in the rad doesn,t cut it with a shift kit

Kick-down cable [passing gear] in need of a little adjustment cable probably streched a bit ,also hope you have a trannie cooler if not you will cook your pump large o-ring external seal , the one in the rad doesn,t cut it with a shift kit

Thanks for the info. I did not think of that because it does downshift at slower speeds, but I havent checked that cable since I rebuilt the carb a couple years ago. As for the trans cooler, I dont have one installed, but have thought about it. The next thing I do is put a trans cooler.

Just hook it on the orginal lines,and put plugs in rad ,don't cut your lines hook hoses and clamp them on orginal lines, that way in colder temps you can switch it back to run threw the rad cooler up here we see -30's it needs the heat from the rad 4sure