KA-thunk

Change the fluid. Back off on the throttle pressure. Or could be Line pressure is to high. (Allen bolt on the valve body) If your shifting it manually it will have overlap if your starting in low/1st on the gear selector .
Pull out with the lever in drive and then put the shift lever to low after the car starts moving. You'll find that it will shift different manually if you do this. High Line pressure will cause your symptom. I had a truck like that and it really clunked into the next gear if you left off the throttle to much as it was ready to shift.