Open loop while crusing

I have a 1987 El Camino with late model 350 and using the computer controlled carb. While crusing 45 to 50 MPH I get a code 13 on the ECM. It will clear if you accelerate. Engine is equipped with headers and the oxygen sensor is in the pipe after the collector. Scanner shows every thing worknig proper when in closed loop. I have held engine at 1500/2000 steady and watched it go to open loop. I feel the oxygen sensor may be cooling off at cruising speed. Would a heated O2 sensor cure this possibly?. Apoligise for the long post.

Did this just start????
If not I would try a new sensor as they are a common failure, But if this has been an on going problem, I would move the 0-2 to the right side header.....

hi in your post you didn't say what kind of t\stat you have in it ? you have to let the engine run at 160 or more in order to let the computer go into closed loop, sounds like the engine is cooling down too much at cruise, and making the computer go back into open loop.