Copied from mye28.com:
Well, I drove the car to a friend's house today to get that fuel pressure test done. He has been a technician at the BMW stealership for about 30 years now, so he has worked on most BMWs from the 1980s on up.
I showed him what it was doing, the 5k RPM rattle or stutter or whatever it was. Turns out it was a rattle. He revved it up to 5k and heard the noise, then took it to the rev limiter just to see if it would go...nothing I haven't personally done before. After he did that, it started running REALLY sh*tty. He said, "Man, this thing's got a miss. It might have jumped time, we need to get the valve cover off and check it out."
Took the valve cover off and lo, a rocker arm had completely broken in half. What luck, this was my only transportation for the time being. The cam lobe for that particular valve was very worn, with a nasty ridge on either side of the lobe. All the other lobes were fine. Now, I understand that this could have been causing a rattle at about 5k RPM. I find it a little harder to believe that it would cause a sudden and major disruption in power under WOT. I find it really hard to believe that a rocker arm can just snap like a twig for no apparent reason, but it did. I would certainly like to know why, though.
The car had not jumped time as we thought...but the timing chain was really loose. It was just a matter of time before something happened anyway. Does ANYONE have any idea what might have caused a rocker arm to just break like that? It sounds like it was on it's way out anyway and the high revs were just the straw that broke the camel's back. Any suggestions or advice would be appreciated!