Or it could be the TV cable isn't adjusted properly.
I know the book says that you should adjust it so there is 1/8" space between the end of the slot and the throttle attachment point -
BUT that's for STOCK IP adjustments...
As soon as you change the high idle screw setting - or remoe it like I did - your throttle move a lot further than before AND so does the TV cable....
SO - now you're pushing the TV lever more and the trans is trying to change it's shift points. BUT - the trans shifts into 4th (O/D), then the load changes on the input shaft and the trans shifts out of 4th - BUT the TV cable says to shift INTO 4th . . . . . etc,etc,etc...
Vicious circle and the poor trans is caught in the middle constantly shifting.
BAD for the trans.
After my rebuild my pressure and shift points changed so much that now I actually preload the tv cable 1/4" so the trans won't shift into 3rd at 22 mph or worse - not shift into 3rd at 60 mph - 3400 rpm....
I adjusted the cable so the tv lever is as far as it can go. THEN I changed the shift points internally using the valve body screws -
like this :
http://i147.photobucket.com/albums/r309/DZL_Damon/89%20D250/IMG_2488.jpg