Error Code 42 on a 1991 sierra

Every now and then when i start my truck, the service engine soon light comes on, and as soon as i shut it off and re start it goes away. When it is on it seems to idle a bit rough. The error code is 42. I have replace just about everything under the hood, even tried a different ecm. But no luck. This may soud stupid but i notice it happens more when it is damp or has rained. Anyway any suggestions on how to fix?

Thanks

you might try this link..I'm sure you know code 42 is EST (electronic spark timing)..maybe it'll help..
http://www.fieros.de/en/v6help/code42.html

Best of luck,
Steve