These drop into OD around 45mph, so that's normal. I'm not a 4T65E guru by any means, but I just changed out the fluid/filter on my 00 GT and installed a Transgo shift kit and so far it has really made a difference. Not only are the shifts quicker but it seems to be much better once the TC locks up as it used to fluctuate whenunder a load. Maybe check out tripleedgeperformace.com for the issues with it not going into gear correctly. There is a lot of info on there that may help you out.