The plastic box near the throttle body connected to the ISV on OBD1 VR6 motors is the ISV damper. It keeps the ISV a little quiter. Without it you might hear a slight whistle, whine, or airflowing past the ISV when the ISV is in use.

OBD2 VR6 motors have the Idle Stabilizer Valve built into the throttle body, so this quickie how to does not abbly to newer VR6 motors, or 24v VR6.

THE CAUSE:
the ISV damper is right above the exhaust manifold and gets pretty hot. Well, that and time allow the idle stabilizer valve damper to crack. I've hear reports of them blowing apart also. Anyway, they get weak and break.

THE FIX:
Remove it. Take it off and chuck it. The volkswagen dealer can get you a new one, but whats the point? If your engine cover is in the way, remove it first. Take the ISV damper of, and completely remove the two hoses connected to it. Leave the Idle Stabilizer Vale on. Take the longer of the two hose and cut it so that it connects the ISV back up without the use of the ISV damper box.

All Done. Go Drive.