Perhaps the solution is an additional temp sensor for the lambda system to prevent it from taking effect until the engine has gotten warm enough.

There already is such a sensor. It's way at the rear, below the CIS (Idle System) sensor, which has 2 wires. The Lambda sensor has just one (Gray) wire ith a push-on connector.

It's supposed to put aground on CU pin #7, keeping it in the default (cold-running) Duty Cycle until the coolant gets up to about 60°F.

It may be faulty and letting CU depend on the O2 sensor signal before the sensor is ready. (Or the O2 sensor may just be not ready when it should be?)
--
Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Sold 5/03)
'83 Turbo 245
'76 244 (lasted only 255,000 miles)
73 142 (98K)
'71 144 (track modified--crusher bound)
New 144 from '67 to '78
Used '62 122 from '63 to '67