My 00 Z24 4 speed shifts harsh from first to second if you driving it at part throttle or full throttle, If you drive it like a grandma it shifts fine. I didn't notice this on the test drive because I didn't floor it on the test drive.

I have read on here somewhere that an IAT sensor can cause this, but I'm not getting an engine light for that. Try it anyway?

Are the transmission mounts sloppy in these cars? If you give it a little burst of gas, say to pass somebody, when you let off the gas you can feel the transmission 'disengage'.. best way I can put it. Its not rough or harsh, thats why I'm thinking its the mounts.

If you go full throttle from first to second it usually shifts a bit slow and like the line pressure is jacked up. It sometimes 'shudders' though..

I got a feeling I'm going to have the transmission replaced or rebuilt but I hope I don't. Theres only 91,000 miles on this car..

mine does the same thing an i have an rebuilt tranny my top an lower motor mounts are shot its common go 2 wwww.rksport.com an buy sum solid motor mounts thats what i did an it stopped