1991 z28vert

Hey everyone!

I have a 1991 z28 convertable with a LB9 and a 5 speed. My problem is when im driving and changing gears my car wont drop to idle with the clutch pressed in. Car idles at 700 when its at a stop but as soon as i start rolling ie. give it gas in 1st, shift at 2500, it hangs around 1200 with the clucth fully pressed in. but when i get to a stop sign it drops back to 700. it does this cold and when its fully warmed up. like i said it only pulls this crap while in motion. the other thing is my speedometer is about 15-20km off @ 100. car is stock nothing has been modded, original gears.

could it be the speed sensor screwing things up?

i have no ses light and not getting anycodes.

maybe someone here has expierenced this and can point me in the right direction

thanks in advance.