Possible transmission problem starting.
 

I have a '92 400E. When I put my car in gear, from P to D, it will go in fine giving that little "thud" that it always does. But, sometimes when I start to push the gas pedal down the car moves about 5ft. then gives another "thud" like it's going into gear again. This doesn't happen all the time and when it does the harder I push the gas pedal the harsher "thud" happens. It worries me because it seems like if I push the gas to hard it might throw out the transmission. Other than that there are no problems whatsoever and I changed the fluid about 6k miles ago. Could be the start of a serious problem?