I have a 2005 300C and it started shuttering in reverse and the shifts were .... strange so I took it in and it was the O ring leaking and low fluid. I never, ever saw a spot on the concrete.
The also flashed the software to the latest. I haven't heard of many failures except when approaching 450 HP at the wheels and then the clutches wear out. Good luck and being a 300 they should know how to work on it.

Les