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Backfire, misfire

I have a 1989 Chevy Silverado 3/4 ton pickup. It has a 350cid engine EFI, 5-spd manual trans, 117000 miles. After it sat about a month, I started it up, it drove fine at first. After it warmed up it started to miss and backfire kinda quietly. Now, whenever the engine is cold, it runs fine, once it warms up same problem as before. Oh yeah, no black smoke when cold or warm. Any ideas?

Any ‘critters’ make a permanent home or devour the air filter and camp out in the intake?

Check the AIC valve for any chewed wiring or nest in the throttle body.

Something is messin’ with the air/fuel or the spark during the warm-up period.

Nope, no critters, no nests, no relatives. Me, no clue.

Check for moisture in the distributor cap.

Perhaps a faulty engine temp sensor?

Computer getting a ‘cold engine’ message?