2002 GMC 2500HD EX CAB tranny problems.

We have a 2002 6.0 4x4 GMC w/ 108,000 miles and about a year ago it acted up about two days and then it went away. Well now the troubles have showed back up. I have changed the fluid and filter about every 35-40,000 miles and the fluid is in good shape. What happens is from a cold start it takes a little more throttle to get the tranny to kick in and the truck to start to move than usual. Then the tranny shifts harder than usual through all gears. After warming up it almost goes away and acts normal. It is all stock and from factory from a standing start it takes a little RPM to get going because of the tow package and all. Thanks for your help. Flyguy.