No way for me to know what's wrong with it, I have no information to work with. Take it back to where you had it tuned up or if you don't trust them anymore, take it to a BMW specialist and get it fixed.
If you want to find a specialist, here's a directory ink for you:
Sounds like either an ignition wire isn't properly seated on a spark plug, or a spark plug itself is either bad, or improperly installed. Not a big deal either way ... just make sure that all the wires are tight in all the chambers, and double check your plugs.
One more thing to look for ... make sure there are no foreign objects left behind in the chamber!!! A carless mechanic could have left a present behing, causing the wire not to be connected properly, probably arching, and mis-firing -- which causes the sputtering.