Help with 88 Z24

Hello all, I am in some very big trouble. I have a 88 z24 cavalier and when I shift gears or push in the clutch, mainly when shifting from 2nd to 3rd the motor goes to 7000 rpms. Car idles fine. It seems as though it gets worse the longer I drive it. It doesnt do it all the time, I can drive it around town and it wont do a thing for like 15 minutes, somedays it does it right when I start it and go. I have had the throttle sensor checked, it was ok. I checked for vacuum leaks, seen nothing. I did do a code check on it, came back code 24. Which was speed sensor, but said disregard if car was not in motion and said check throttle sensor. I had advance auto look up a speed sensor when I had my throttle sensor checked. Nothing on file for a speed sensor for a manuel transmission. I did put a new clutch with pressure plate in it this past summer. I'm out of ideas. Please any help would be appreciated.(Yes, I have my foot off gas). Thanks, Shawn