Due to the RPM's staying solid I began to suspect the trans shifting out and in gear quickly, so I switched to manual mode, but there was no change.
It always idles perfectly in drive (at a full stop) or in park. It also revs perfectly in park, as well as in drive when at a full stop and holding the brake and revving just a little (but not moving).
I have also watched the RPM's very closely and have caught it doing it when changing gears and the RPM's are dropping quickly (as normal)...and it drops perfectly smoothly so I am getting some hope that it may not be the engine unless the RPM gauge is just showing an 'average' rather than a live reading.
This all has made me start wondering if it could be something with the traction control as it feels and sounds a lot like the hill descent does, and only happens when it is moving. The sudden 'loss of power' and shudder made me suspect misfiring before but a quick tap to the brake on one wheel could feel similar.
...but then that makes me wonder what could do that without registering an error?