I have not driven your car but it seems to be a common problem with 300e's when milage is above 150k.
I would suggest that you have a trans shop flush your trans and change the filter and fluid. It may be a small price to pay if the problem turns out to be more serious. But if the fluid is old, tired and dirty, it can cause problems....
My bet is that this will solve your problem, It worked for me!!!
BLACK GOLD WITH CREAM INTERIOR, SPOILER, AIRDAM
BRONZE WITH CREAM INTERIOR