Can't say that this'll fix your problem, since your symptoms of rough idle and RPMs dropping when you disengage the clutch is not what I was experiencing. But for me what eventually made my P1124 P1126 and P0446 codes go away (at least, so far) is replacing the tank vent valve,
996-110-129-06, that's under the driver's side intake manifold.
Starts, Fires Up, Dies... - Pelican Parts Technical BBS