I picked up my '04 330ci in December and on the drive home from Chicago I also noticed the surging under power. When I got home I filled the gas tank and added two bottles of Chevron Techron fuel injector cleaner. This cleaner was recommended by name in the owner's manual for my '94 325si. I used two bottles as the fuel tank size was in excess of the gallons treated listed on the label.
Haven't driven much since then, but last week I ran it up a little and it began missing on one cylinder badly and the check engine light came on. As I was only a couple of blocks from home I kept going. Restarting the car a few minutes later found all to be well and I have had no problems since. Surging has not returned.
I'm pretty sure it was just gummed up injector nozzles and the cleaner loosened a lump and stopped it up as it went through.
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