If it's not the O2 sensor directly, my next guess is an air leak before the throttle (not likely on K-jet, but check the rubber coupling) or a vacuum leak after.

The the extra air may not be a problem until the rich cold mixture leans out. Then the O2 sees lean, the ecu goes rich to compensate. O2 may be slow so it says lean too long, etc., etc.

Here's a way to hunt for vacuum leaks:

Take the nozzle tip off a propane torch and replace it with some snug fitting rubber hose about 2 feet long. Practice with the valve to get a moderate gas flow (not a roaring blast).

With the engine at a warm idle, open the gas valve and poke the end of the hose close each injector for a couple of seconds. If the seals leak, you should hear the engine smooth out or rev up when the propane gets sucked in and burned.

Do the same around any other suspected areas, like hidden vacuum hose ends and the intake manifold gasket area.

To block any breeze from the fan, lay a piece of cardboard from the fan shroud to the engine.

--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.