Just purchased a '99 Z28 SS with 64K and noticed that when I am between around 41-49MPH the car is "jumpy" almost like it can't decide if it wants to go into overdrive or not. It is an automatic transmission with a Drive and Overdrive option. When I am drving in just 'D' it does not do this. Any ideas?

 

Your trans is on the fence of trying to be in D or OD, which is why it keeps hunting as your speed varies. I never put my TH700 or 4L60 into OD until I hit the highway, as overdrive isn't needed or wanted when the car is driving in the 40's speed range. Being in overdrive at that "low" of a speed lugs the engine down too much and hinders performance, and as you're finding out, can be annoying too.

 

That makes sense. Thanks!