Jasenjo, please ask another dealer or call GM'* customer service if necessary, you will be soooo much happier after they apply the software update that is included in the Garage Shift Stall TSB (I have those numbers Custom88 mentioned 01835 written on some old paperwork so that may be it).

You shouldn't have to live with a glitch that is such an easy fix!

It can't be that obscure, my dealer knew exactly what it was and cured it the same day.

I'm reviving this old thread because I am having this exact problem with my 2004 Buick Lesabre, which is the same body code and same almost everything including drivetrain as the Bonneville.
Does anyone have any information on what they do under TSB 01835? The last post in this thread mentions software, is this TSB only a software update?

My Tech II scanner also shows no codes and the rest of the transmission functions okay aside from the expected slow shifts which I remedied with a shift kit.