As mentioned, the car needs some maintenance. Valve adjustment, filters and fluids. I would GO BACK to a plain old Bosch Super plug, that is what works best in this engine.
I would replace the O2 sensor, buy the $12 socket tool, makes it easy. Could be lots of other things. Wires, cap, rotor, fuel injectors, ICV....
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5