96 gtp with dohc misses

96 gtp needed a water pump, so i let the shop replace the timing belt since it was getting some miles on it. this was not a gm dealer. got the car back and it was running rough and not idling right. took it back. they replaced the idle control valve and mass air flow sensor. still had problems. took car to pontiac dealer. dealer said timing was off and reset cam timing. car is still not running right. when accelerating at a slow to moderate pace, when the speed reaches ~35 to 40 mph, SOMETIMES it starts missing or skipping. i can accelerate quickly and do not get the missing problem. i have replaced the fuel filter, cat converter, wires, plugs, and tps. the fuel pressure has checked out ok. the dealer cannot get the car to miss for them. have run injector cleaner thru tank many times. sometimes when the car is started, black smoke will come out the exhaust. there is no SES light on. the motor just seems like it isn't running right at speed either. it might be missing at higher speeds also, just harder to feel it. this has been happening for some period of time. suggestions are welcomed.