Chevrolet: 700r4 transmission, 700r4 trans, modulator valve

Question
I have a 86 corvette with a 700r4 trans.  It won't shift into 3rd or od.  It did this while I was driving.  I passed a car and after the downshift it wouldn't shift into the gears.  It will shift into 3rd, but as soon as you accelerate it down shifts.  If your in 3rd you can manually downshift.  The tranny fluid is red and clean and doesn't have a burnt smell.  I have done the adjustments at the carb. to adjust the downshifts and they are fine.  Could this be the modulator valve sticking? and how does it come out of the tranny?  Thank you

Answer
Paul;
It could be the computor , or in the valve body. I would take it to one of those 24.95 tranny places as they will check it all out and tell you what it is, this way you will not spend a lot in taking it all apart, as it could be a few things. ok