Possible Causes & Corrections:

Ensure EGR valve is not always open.
Check for proper valve timing.
Check for engine vacuum leaks. Ensure engine is tuned to specifications.
Check for faulty air injection diverter valve or check valve.
Check engine for sticking or leaking valves.
Check for fuel or water in vacuum hose to MAP sensor. Also check for restricted hose.
Using Spark Tester (ST-125), check available output voltage of ignition coil.
Check for crossfire between spark plugs, distributor cap and spark plug wires.
Check for an intermittent ignition system problem.
Check for erratic spark timing or distributor reference (RPM) signal.
Ensure initial ignition timing is properly set.