I agree that it is most likely a slipping clutch, although if it is slipping that badly you would think you would smell it.Strange, not sure about the loss of power thing and drop in MPG. Would be worth checking for codes like @Truchman said.
If your RPMs are staying consistent but speed is dropping then there is a problem in the drivetrain somewhere. I would most likely attribute it to the clutch slipping since it has been a known issue now. If your RPMs are dropping with your speed then that would be a loss of power.
The engine sound makes me wonder of it is an intermittent misfire or something similar. It should throw a code, but it may stay as a stored pending code if it stops.
Maybe the knock sensor is retarding the timing under high load?