[Chocolate Apocalypse]

Is it just on the 1-2 shift?
At the very least I'd check the fluid in the clutch master cylinder and replace it if it's too dark. Also, make sure there is enough fluid. The stock hydraulics are terrible when it comes to quick, WOT shifting.
If that doesnt do it I'd suggest new hydraulics w/the drill mod. I did this on my last '99 Z and was still having issues on the 3-4 shift at WOT. The pedal would get very light and sometimes didnt want to shift. At the extreme is the Tick Performance master cylinder, everyone raves about it and if my current car was a stick, I'd have it. It's a little pricey but apparently is worth it.