[MJN]

Sounds like airflow restriction to me. Couple of suggestions. You may have already tried this but pull the air cleaner(s) off. The small air cleaners typically used on Cobras are very restrictive. Second, try pulling off the side pipes. The typical glass packs used in the side pipes can be extremely restrictive. If you can do this while on a dyno it may help to uncover the problem. My torque curve sounds exactly like yours and my problem is the side pipes. With my cam and heads the torque curve should hold flat until around 5000rpm but as you can see it takes a dive at around 4000rpm.