Audi A4 clutch sticking

[jomool]

My A4 clutch is sticking intermittently - sometimes when I press the pedal it doesn't return and I need to pull it back up. I've had master and slave cylinders replaced with no improvement. The interesting thing is, if I'm in neutral and clutch is stuck down, when I rev the engine it pops up immediately by itself. What could that be? Appreciate your help!