Transmission shifts lag when cold

Have a 2000 GMC Sierra 5.3l, not sure which tranny it is, but when engine and tranny are cold the 1-2 and 2-3 shifts seem weird. It will make the shift and immediately after the truck will lag or hesitate slightly and then run great till the next shift and do it over again. This only happens when cold and when under light to moderate acceleration, if under heavy acceleration the problem is not noticeable. During normal operating temperatures, the shift points are smooth. Any ideas? (besides driving with a lead foot)