96 gtp, 3.4L, having problems, repair history slightly long

96 gtp, 3.4L, 4 cam engine, 100,000 miles. water pump was leaking. had shop replace pump, timing belt, and spark plug wires. car started missing, idling rough, and would really miss around 35-40 mph, BUT not all the time. shop replaced maf, iac, manifold gasket, spark plugs, replaced fuel filter. checked fuel pressure, 45 psi. still had problems. took car to pontiac dealer. they retimed the cams. car still had intermittent problems. would sometimes bog down when accelerating from a stop, still missing around 35 to 40 mph. replaced catalytic converter. car still giving trouble just not all the time. took pontiac mechanic for a ride, driving the same roads and the same way that gave the missing and bogging. nothing happened!! dealer cannot duplicate the problem so they don?t know what to do. no service engine light ever came on and no computer codes. i have crawled around under the hood looking for vacuum leaks and loose wires, but nothing obvious. this is really happening, not our imagination. what to check next??