You have described what I have been fighting for months. The valves have been adjusted, timing checked, even put in a rebuilt (vacuum) distributor, corrected the PCV system, new springs, no air being sucked in anywhere else that I can find and had the carbs rebuilt and throttle shafts done twice as there was a leak. After all that, what you described continues. I am waiting for replacement SU's right now and if they don't do the trick I will be lost and be forced to actually take it to a shop. When you find your solution, please post it. It will be interesting to see if we are fighting the same demon.
DC