I have a 2005 Chevy Impala (71,000 miles, 3400 engine) that has an initial slip in the transmission when accelerating from a stop. As you take your foot off the brake and step on the accelerator, the transmission hesitates, then kicks in with a sudden, stiff clunk. Subsequent gear changes and acceleration are normal. This always happens after the car has warmed up for 5 or 10 minutes, and can be avoided if you are VERY careful and step GENTLY on the accelerator. There was no noticeable metal in the transmission fluid, but I changed it anyway, to no avail. The car gets very good gas mileage and otherwise runs well. Any advice?