be the first report of this I've come across if that proves to be the case.
But consider this: At steady cruising speed only about 40 or 50hp is needed so the engine is not making gobs of torque. For the clutch to slip then engage -- if this indeed accounts for the varying RPM -- and do this repeatedly is not what I would expect. However, thinking out loud, if there was some hydraulic/electronic circuit malfunction that resulted in the clutch being slighly disengaged then engaged ever so slightly this might explain the behavior. I would think in this case though there would be an error code and a CEL even though the error wasn't related to the engine. Those PDKs are as much computer as tranmission.
One "test" you can do is with the engine/drivetrain all warmed up is take the car out someplace and put it in 7th gear. I'm not sure how you can avoid the transmission downshifting but under the same conditions -- speed/RPM -- give the engine some throttle as much as you can while avoiding triggering the downshift. If the clutch pack was slipping under a steady load it should slip when you increase the load and the slipping should be rather obvious.
You can also under the same conditions put the transmission in 5th gear. This gear uses the same clutch pack at 7th. Then at about the same RPMs give the engine as much throttle as you can without triggering the auto downshift feature.
You are looking for the engine RPMs going up at a rate or to a level that does not jive with the vehicle's speed.
My WAG is the behavior is not coming from the PDK but from a fueling problem.
Oh, you can try this. Do an e-Gas calibration. The steps should be in the car owner's manual. From memory at least for my cars: With the key off and your foot off the gas pedal turn the key to the first position the on position and leave it there for 60 seconds. After at least 60 seconds turn the key off. Leave it off for at least 10 seconds. The next engine start the calibration will be complete.