Anyone ever stick an oscilloscope on the coil primary to see charging current and voltage? Watching the voltage on the coil ground and charging current under load at speed may show a defective waveform, and probably condemn the DME at that point. That transistor is less than $5, and there was a thread on replacing it just a few weeks ago.
Broken valve springs typically show up as a high-RPM misfire/false low rev limit. Also a possibility, but yeah, the VC R/R starts at $500 at a dealer.
How much do you like this car? As in: How much are you willing to spend to continue chasing this problem? Are you getting this done under an extended warranty?
E92 335i/6MT Dinan Stage 3 intercooler, oil cooler, intake, flash.
E28 M5, R171 SLK350