GM-GMC: 1990 Chevrolet 1500 Small V-8, (305?), silicone grease, style engine

Question
I have a 1990 Chevrolet 1500 that for the last couple of years has been inconsistently misfiring and turning on the check engine light.  This is an old style engine computer requireing blink counting, and the results have been inconclusive.  The check engine light will (sometimes) go on after driving 10 to 20 minutes, but once the engine is turned off the light stays off.  I have replaced the plugs, wires, O2 sensor, and had it into a shop several times.  Some times it seems to help, but the problem seems to return in a month or 2, but being intermittent - it is tough to tell.  I can't seem to get rid of the intermittent misfiring.  This is beginning to sound like the computer to me.  Is there an inexpensive way to test this theory? Or is there something else I should check?  Could it be in the pollution controls, vapor canister or gas cap?

Answer
This is one of the reasons I drive mechanically injected diesels!

Yes, your problem could be from any that you mention.  It may also be from loose and/or corroded connections between the computer and sensors.  I would disconnect as many of them as possible and inspect them for corrosion, recoat them with silicone grease (if it is used on the connection) and reseat the connectors firmly.

Have you changed your fuel and air filter?  There is also a filter in vapor canister.

You might also want to try and get the code to display immediately after the engine light comes on.  It should tell you what sensor or circuit is causing the problem.

As an outside possibility, check the sealing of your intake manifold to the heads.  Use some carb cleaner and spray it along the manifold where it meets the heads.  A change in engine speed indicates a leak.

I am sorry that I can not offer any more than this general guidance.