This fault is the reason I ended up buying a Beamer. I had this fault on my previous car a 3.0i 24v Vx Senator.
I ended up replacing all 7 Ht leads, 6 spark plugs, cleaning the rotor arm, cleaning the dizzy cap contacts, fitting a new crank angle sensor, fitting a new lambda sensor, stripped and cleaned out the throttle body, cleaned out the idle control valve, resealed all air induction hoses and checked and cleaned numerous electrical sensor connectors.
I also ran through a tank of fuel with redex fuel injector cleaner.
None of the above cured my fault (but each time I did something the engine seemed to be more responsive and smoother but not cured) but thats not say it won't cure your fault.
I had heard that it may have been a fuel injector seal or another seal in the inlet manifold system that was leaking and causing the flat spot. I didn't want to throw any more cash at the Sennie so I got rid of her for my E39 523i SE.
Clean and check out all what I have said and hopefully it will cure it and give it a damn good thrashing to try and burn off any coke stuck to the head and valves.
Andrew