I thought my 99 monte was going into 4th way to early. The trans shop I took it to let me ride as a passenger - I was holding a scan tool that showed what gear the tranny was in. Turns out that what I thought was '4th gear" was actually the torque converter locking up in 3rd gear.

The tranny would go 1 - 2 - 3, then shortly after 3 gear engaged, the TC would lock up. It felt like another shift, but it wasn't. It was a big drop in rpms too. Happens at 37-38 MPH under light throttle. Then 4th would engage at 45 MPH or so.

I believe the 99 also has a 4t65e.

 

97-99 Z34's all have the 4T65E, like you expected. All other 5th gens don't.

 

I have to correct my previous post. My 99 Monte w/ the 4T65E has a 3rd gear torque converter lockup at about 27 mph, not 37 mph. That's why I also thought mine was not shifting correctly. It shifts like this under light throttle:

2nd - 12 mph
3rd - 22 mph
3rd - TC lockup - 27 mph
4th 45 mph

The third gear tc lockup feels like a shift.