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2009 Merc-Benz CLS 63 AMG, with approx 20,000 miles.  Still under factory warranty.

Bought it about 2 months ago, and it came to me with this issue, despite it being a MB certified vehicle.

The issue is that it has a subtle vibration/shudder/hesitation from around 65 to 80 mph.  Imagine if you're driving a straight-drive and you put it in too high of a gear and the car gets bogged down and shudders/vibrates/hesitates....that's what it's like.  

It's always worse under slight acceleration and SIGNIFICANTLY worse with acceleration going uphill.  It always happens under load.  And it goes away if I do a "kickdown" by pressing more aggressively on the gas and making it go into a lower gear (7th gear down to 6th).  And it again goes away if I manually shift using the paddle shifter to do the same (7th to 6th).

The dealer had the car for a total of 4 weeks.  They've replaced the tranny mount, which had no effect.  Took it back and then they changed the torque converter and one of the valves in the valve body (not sure what the name of it is, but they said that it's the one valve that should be changed whenever the torque converter is changed).  No improvement whatsoever!

Now they are telling me that they won't even further investigate the issue until I drive the car 1000 miles.  They tell me that the car has to adjust and make adaptations and the issue may eventually go away.  Frankly, I'm not buying that.

I'm at wit's end.  Any ideas/suggestions/advice?

Thanks.  

The next time it's happening use your left foot to apply the brake pedal slightly. Just enough to activate the BOO switch {brake on off switch} without actually braking the vehicle. If it goes away then the problem is still in the TCC system {torque convertor clutch}.  

Tried that today.

No change or improvement.

What else could it potentially be?

Thanks...  

If its not tcc related then i would think its an engine performance issue. Either a timing,fuel or ignition problem of some sort. At this point i would start keeping a diary of service visit dates. phone calls .who you spoke to and what was said. Look up the lemon law for your state so you know where you stand.  

I have to agree with 440t4, however, on some foreign vehicles, the BOO, does not cancel the lockup with the converter.  That being said, try lifting your foot off the accelerator while cruising and re applying.....to see if the problem still exists.

EVEN IF IT DOES, it does not mean that the converter clutch is the the problem, (however, it is most likely, given your typed description), when in "lockup", ANY engine misses, fuel problems etc. are much, much more noticeable.

And heed what 440t4 says about keeping a diary, this will save you a lot in future expenses.