It could be the injectors, or the IMV, or the fuel pressure sensor, or a myriad of other things. You're pissing in the wind without fault codes.

If you get P0251 and P1211 and no injector codes, it could still be the injectors. If you do get an injector fault code then I'd 100% take it to a specialist. It can be very difficult to get F-Super to take the code!