Hey guys, similar thing when I bought my 4.6is X5, was never wowed by it by performance during the test drive but I believed in it :-) On the way home at high RPMs the check engine flickered and the truck lost power...ran code and it was missfire cyl1 fuel cut off..after running slow for a while it ran fine until i pushed above 5000 rpm. So what could affect cyl 1 only? change injectors (all 8), plugs, and coils and? ran better but still happening...
Come to find out the CAT on bank 1 was all clogged up.. weird how it wasn t affecting all cylinders...
Anyway, tap your cat(s) with a fist or mallet, if you hear any sound i d change them (or it if only one on a 6 banger). $1200 a pop!! so I got a set of high flows aftermarket and oh boy...that wake it right up :-)