[RUN GMC]

I own a 87 GMC 350 tbi/700r4 and when I first got it I had the same problem over drive riding out longer than it should and disengaging at about the same as yours.....what I found out was the trans fluid was low but read ok on the stick because it foaming and causing torque convertor shudder......to tell this is when you pull your trans stick check after driving it for any bubbles no matter how few they are and if so add an 1/8 of a quart of trans fluid.....keep doing this and you should feel improvement in the driving and the shift and lock up....mine locks up at about 50 mph and disengages at 45 mph......I did this over a period of 3 days mostly highway driving before mine stopped at the later stage moveed to the more normal range......hopes this helps just look at your trans dip stick closely you will see a difference in the fluid coating it......