Nissan Repair: largo Van, throttle position sensor, air flow control

Question
We have a 1996 four wheel drive limited, and when it runs right it goes very well, we have a problem that I have talked to mechanics about, and no one seems to be able to give us a straight answer about, the  problem is sometimes as we are driving along the van loses power like we are running out of fuel, now if you put your foot to the floor on the gas peddle it leaps into life and takes off like a jack rabbit now I must say our van has 182,000 k's on it, the motor is a KA24DE petrol(NW30) I have replaced the spark plug leads, distributor cap, rotor button, which were all well worn as well as the top timing chain and guide along with the tensioner now this motor has a bottom chain as well from the crank to an intermediate sprocket that joins bottom to top chain, tho I have no noise coming from the bottom chain could this be part of the problem? I have ruled out fuel pump as it leaps into life when you put your foot down, I have been told that it could be the air flow control meter, I almost forgot I have replaced the knock sensor as well it had cracks in the coating which is not good Nissan said, Can someone  help me please this problem is driving me crazy, Thank you all Ally any help will be great fully appreciated .

Answer
Ally,

First check the distributor for oil on the optical pickup and receiver.  If this is good then check the throttle position sensor for normal operation, with a volt / ohm meter to make sure there are no bald spots.  That is, the resistance is in good working order.  The chain should not be causing the problem and if there is no noise and the tension is within range I would leave it.  The mass airflow sensor could be a problem and the difficult part is that they normally do not set a code when they are acting up.  But, at the cost it is expensive to just throw one on the vehicle.  I would first test it after ruling out the distributor.  I have had the distributor problem with oil on the optics too many times to count so that is where I would start.