Chasing 87' TBI idle problem. Included service code

[chevymotocross]

My truck, with the 305 tbi has been having an odd stalling problem. When first started in the day, the truck idles like poo. If I turn the a/c on and put it in gear, it will stall. Once it warms up, it idles good, but if you put the a/c on, and you aren't actually moving, it will stumble and stall.

I checked the IAC valve and passage. It is clean. The Coolant Temp Sensor was replaced last year since I suspected it was that, no luck. I checked for the codes it was giving and I get 3 flashes which indicates a Code 13: Oxygen Sensor issues?

The SES light goes away once the truck is restarted. Or, if it is about to die and I put it in neutral, the light will also go away.

Anyone have any input on this?