hey i`m actually South African and i do believe u about the o 2 sensor.... i think it had some thing to do with the temp back home....
i would check for a miss fire i had a similar problem with my 325i it turned out to be the coils thy had gotten moisture in and as soon as it wormed up i had a lag it would stall out ... try that hope it helps i changed mine out after checking all kinds of other thing and now she go`s grate again... o and make sure your plugs are tight bmw`s are bad 4 blowing them out...