Possible Causes & Corrections:

Ensure EGR valve is not stuck open.
Check engine for sticking or leaking valves.
Check for proper valve timing.
Check for engine vacuum leaks. Ensure engine is tuned to specifications.
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.
Check for fuel or water in vacuum hose to MAP sensor. Also check for restricted hose.