Carbon cleaning should run $800-1000. Not particularly cheap, but not too steep, either. It should be done every 30-40k miles at least. I bought mine with 46k, did the cleaning at 47k, and the buildup was horrendous. I noticed much smoother running and a little better power at the top end. I manually scrubbed with gun cleaning brushes and carb cleaner. Forget that! It was a mess and a PITA. I bought the BMW walnut shell blasting tool to do Mini and BMW DI engines. Maybe I'll run a special on cleaning services next month...
Wow, I didn't know injector prices had come down that far: 13 53 7 585 261 is $135 retail, available from Tischer BMW for $108 (20% off retail) each.
Vas, try replacing the coils - all of them. If they don't fix it, you're not out a whole lot. 12 13 7 594 937 $60 retail, $48 from Tischer.
E92 335i/6MT Dinan Stage 3: intercooler, oil cooler, intake, flash, exhaust, LW dual-mass flywheel.
Vishnu PWM methanol custom trunk mount, Diffsonline 3-clutch Salisbury custom diff, Ohlins Road and Track coilovers, Vorshlag camber plates, much more...
E28 M5 w/ buncha stuff, R171 SLK350