does that motor have an egr valve?, and if it does, unplug the vacuum line to it, and check this......what happens (if this is the problem) one or more ov the actual egr ports in the intake gets clogged, then sending all that exhaust gas into the remaining unclogged egr ports in the manifold......this causing a lean condition, and making the motor bogg down, at any rpm just depending on the finnike-ness of your motor.....and this problem is completely seperate from the ignition and or fuel system....anyways it takes like 2 secs to unplug the line and cap it off.....its worth a shot just to try it and rule it out
