First thing to check is the AT fluid level. Your Impala might still be under the power train warranty. Check with Chevrolet. Mine is a 2003 Impala and is still covered. Also visit www.mygmlink.com or call 1-800-222-1020
Sounds like a bad seal(s). Seems that the seal(s) shrinks when cold and allows the fluid to bypass with out doing the work you request. Then when the temp warms up the seal(s) expand enough to work but with the "pulling" your describing. One or more may also have a chip or crack in it. This transmission needs either repaired or overhauled, depending on mileage. Get this done soon for it won't last long.