I have a car in the shop now with the same persistent cylinder 5 misfire under load (fine at idle and cruise). Easy to replicate the misfires, 3rd or 4th gear, 2500-3500RPM full throttle. It revs through the misfires and accelerates (albeit slowly since it's stock) all the way to rev limit with little drama. Takes several pulls in 3rd/4th to finally set the SES light and cylinder 5 shutoff.
E93 335i 6MT, 36k miles, no tune, all stock. New BMW Bosch plugs. New coil on cyl 5.
1) Swapped coils 5<->4, customer has already installed a new coil on 5 previously.
2) Swapped injectors 5<->6, misfire on cyl 5 still. Decouplers on both injectors, seals look fine, these are probably replacements from when the car was still under warranty. Tips of injectors looked dirtier than others I've seen but not terrible.
3) Injectors coded properly in ECU
4) Car does not have injectors that qualify for warranty replacement
5) Checked wiring and ECU connections, no issues found.
6) Injector bore and step for the decoupler element look perfect - no galling, damage, or trouble visible. No metal-to-metal contact found on the tip of injector 5.
2AAF Fuel pump plausibility - intermittent, no low pressure fuel system problems found.
2D18 Tuning Protection (Air mass flow max - tuner code, don't know why since it's all stock, hasn't come back on 3 test drives, though)
29D1 Combustion misfires cyl 5, with cylinder shutoff
2F81 Engine Switchoff time, damaging exhaust gas
2F83 Engine Switchoff time, no matching fault symptom
Next step is to bore-scope the cylinder and maybe check for extreme carbon build-up in the intake. Have to get further diagnostic time approval from customer, will continue to update the thread...