Have you checked both the level, and condition of the transmission fluid? Needs checking with engine running in park. probably wouldn't hurt anyway to do a coupleof drains and refill with fresh fluid if you don't know how old the fluid is.
My car doesn't have the "graunch" upon accelerating from standstill, inaudible but is sort of felt through the steering wheel for no more than a split second. Thirdly it almost refuses to accelerate in "I" mode without nearly 50% throttle, often causing the slight shudder" you report, with 157k miles on the clock.
Interested about the Terraclean, thought about it myself but hesitated. Might give it a go.