I know firsthand your pain. I just replaced everything you have. It might be an air pocket in your system. Try this (if you havent already) with the car cold, open your bleed port(s). my 96 has 2 of them. one of the passenger side of the head (on the EGR housing), one on the heater pipe on the front side of the engine. with them open, fill the reserve bottle until coolant is coming out in a steady stream from both ports. close off the ports.
then cap it off and try it.
mine is doing the exact opposite, i can idle for HOURS (literally i have!), but cant drive it more than 2 minutes without overheating.
I suggest heading over to
www.J-body.org there are a few ornary members there, but there is a wealth of info to be found.
Good luck!!