c4 transmission

Get thee to a Tranny Shop

If you pull off the vacuum modulator line there is a screw in there you can adjust to raise or lower your shift points. I would probably replace the valve (one bolt clamp on or screw in) and get a new one. All you need to know is it push in type or is it screw in (I think), and the color of the stripe around the valve (yellow, blue, purple etc). It is cheap and easy to replace, then adjust with the screw where the vacuum line goes in. With all that said though, if that doesn't cure it the tranny is probably on it's way out. Good luck.