Bank shutting down when cold...

My 2002 325i 170k manual starts fine and idles perfectly, but when I go to drive off sometimes it looses power and runs rough - feeling as if I have only 3 cylinders. It will stay like that until I turn it off - but once I restart, everything is then fine. Any ideas what is happening ? When it happens I get a check engine light and lean code on one bank. It happens more often in cold weather or when not driven for a few days, but sometimes will go for weeks without any problem. It seems to me that there must be some condition for which the ECU shuts down a bank, but can't understand how the issue can just go away by a restart...





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