Hey I have a 2003 and I have the same problem.
And I hear the same thing as you, can't find the problem. I think GM should use some of that bailout money to improve their customer service. They know its a problem but yet they won't admit it and they won't fix it. My neighbour works for GM, has an Impala same as yours and he has the exact same problem. So this is very common.
There is one thing you can do, buy a product called Lucas Transmission treatment (under $10). Put half of the bottle in where you fill for the automatic transmission and take it for a drive. Mine is so bad we did that and had to put the other other half of the bottle in. It been ok since the summer, but now it is slipping again.
I'll be watching your answers to see if there is anything that can be done, cuz GM certainly won't own up.