I have a 2002 Sable LS Wagon, 24v engine with 67,500 miles that occasionally misfires at idle - doesn't matter if the engine is cold or hot. It also stumbles slightly when initally accelerating with a cold engine. The stumble disappears when the engine is hot. Cannot detect misfire at highway speeds. Had mechanic run a diagnostic check - no codes. Ran Techron fuel injector cleaner through it - no change. Mechanic found short u-shape hose under throttle body assembly saying it was soft and porous and that it was the cause of the misfiring - not so, it still misfires. Any ideas? Thanks