Hi Haplo,
I had a similar thing happen to mine a few weeks back. I don't know exactly what revs it would change at as my clocks were fooked at the time - now been replaced. :dance:
The auto box wouldn't change up until the motor was screeming - then bang, it would change hard into second gear.
Believe it or not, the culprit was a blown headlight fuse!!! Can't remember which side though - I replaced said fuse and bingo - smooth lovely low reving gear changes - check yours out as it may be the same? Be interested to know if it is this on yours or if anyone else has had this experience.
I thought at first that my box was knackered, but searched as many threads on the site for info relating to faults and probs members have had - excellent info and a big help. I also checked fluid level and condition and all was superb - electrical gremlins seem common on the Z in my very short experience in owning one, most relating to simple stuff like fuses and connections.
Hope this helps a little.
All the best with it,
Nigel