That sounds like it's slipping. When you check the oil level, are you checking it hot or cold? Auto Trans oil levels should always be checked hot. If you forget to do it just after driving the car, there's another way to check the level from your driveway. To do so, open your bonnet, start the car and allow to idle a minute in P or N. Then with your foot on the brake, place the car in D, leave for a couple of seconds, then move to R, leave for a couple of seconds and return to D. Repeat this 4 - 5 times with your foot on the brake (Even if the handbrake is on). When you've repeated this 4-5 times, return to P and check the oil level quickly (Remember - pull out the dip stick, wipe clean then put right in again and pull out) The oil level should be between the two marks on the dip stick.