2003 2500HD Duramax,6 speed transmission: sometimes it won't shift in to or out of gear while engine is running. Engine off everything ok. When this happens while engine is running, tranny in gear, clutch depressed, truck won't roll down hill. last time it happened doubled pumped clutch pedal with engine off, then everything started working fine agian. any sugggestion on what it could be?:help2: