2002 Pontiac Grand Prix Misfire and staling

My 02 Grand Prix SE bogs and stumbles sometimes on uphill acceleration and sometimes at idle after it gets warm. Once this happens it continues until I stop the car and turn off the ignition, wait for a few minutes and restart. This seems to clear the condition until? Sometime sooner and sometime later. My mechanic changed the O2 sensor (bank 1 ) and it was good for 1- hour. Then he changed the fuel filter, the MAF sensor and the fuel pressure regulator. NO LUCK> THIS GUY IS A MASTER ASE CERTIFIED MECHANIC and ain't cheap. Up to 900.00 bucks and no Luck? Could you give me some ideas. An acquaintance of mine said it may be a leaking intake? Why wouldn't a certified mechanic see this? Thanks, I HOPE
Wingnut