1995 BMW stalls

Stomp test is giving me 1221 code an 02 sensor code, changed it today and it ran great for bout 10 backfired and continued on to run good for bout another 30 minutes and the same problem again. What could this be?Really the old 02 sensor looked as if it had been changed before I purchased the car, well it had the Bosch one on it which I assumed was the after market one even had the maf sensor tested and cleaned, got new catalyc converters also still wants to stall. Ok after all of this we've replaced cam sensor and mass air flow sensor also, but once this car warms up once you attempt to drive it will stall(falls back down to idle) at.5miles every time, you can turn car off and restart it and its fine for another.5 miles, does seem to have a strong fuel odor at stall point. As long as the car is cool it runs fine problems begins once its warmed up