I've always noticed a slight slippage in my 300D engine speed/road speed when at 1/2 throttle and full throttle. It seems that more torque being sent through the transmission at the same road speed always increases the engine speed by about 200-300rpm in 4th gear. My other 300D that has less miles and has been treated far better per the service records doesn't slip at all under the same circumstances. Is this a problem with the torque converter or the transmission? Maybe a sign the previous owner did some neutral drops or something?
Is there a way to eliminate this with special fluid? I changed the fluid/filter about 9k ago when I bought the car.