I've got a TH700R4 tranny in my truck. The damned overdrive is driving me nuts. If I drive right at 40mph it wants to downshift/upshift constantly. Has anyone used one of those valve bodies to control lockup? Has anyone used a B&M shift kit?
I haven't really dealt with 'older style' trannies before, but have you checked your down shift cable? I think there is also another one, can't think of the name, but they might be out of adjustment, and need to be 're-tuned'.
a shift kit will do you no good for youyr problem. the first thing i'd do if i were you is to recalibrate the TV cable. this is the cable going from the tranny to the throttle linkages. there will be a cable from the throttle linkage that leads to a plastic "nipple" that sticks out of the TV cable. there is a little D-shaped button on this. if you press in this button and then push the nipple in all the way. you will be at 0-setting. then stomp the gas pedal and the nipple will be pulled out of the TVcable housing. this is the base setting. i purchased a new TVcable from the dealer for 24 dollars and did this with mine. since there is no stretch in the new cable yet, the base setting shifts are late and firm. for every one click the nipple is moved back in the shifts will decrease in speed about 2mph. after playing with this you might be all set. failing that, your torque converter lock-up might be bad and you're feeling it lock and unlock. a manual bypass with a toggle switch can cure this for you, or you can purchase a vacuum controlled switch from i think transgo...but I'm not sure. I'd have to dig up my paperwork and see where i got mine from.