Intermittent Check Engine Light

Inspect CHECK ENGINE light wire to ECM.
Check ALDL connector wire to ECM for short to ground.
Check for electrical system interference caused by defective relay, ECM-driven solenoid or switch. These can cause voltage spikes. Normally, spikes occur when faulty component is activated.
Check for improper installation of electrical options, such as stereos, car phones and fog lights.
Ensure CHECK ENGINE light comes on, but does not stay on.
Connect a voltmeter to a suspected circuit and ground. If voltage reading changes as CHECK ENGINE light comes on, the problem is located in that circuit.