The harsh downshift could be resulting from driving style. The trans computer tries to decide the style of shifts that you want from the way you drive. If you drive slowly then ask for aggressive shifts the computer doesn't respond quickly. Try to shift the trans manualy durning your next trip & see if the car will keep up with you. The trans should never go into "Neutral"!! There has been several mods to both valve body & ECU. Try having the shop foreman at your local dealer ride with you to see the problem. To fix this type of problem a technician must be able to duplicate it. The ECU should be able to provide any slippage info.
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR