Triumph Repair: VitesseII violently rough under load, dial gauge, lumenition

Question
1971 Vitesse 2lt Mk2 with 6 y/old lumenition K & N's 6AC needleson CD150 Strombergs. Have had problem for 2 yrs off & on now constant. It bucks and intake backfires under acceleration. Will run ok 'backed off' but a complete pain!
I have extensively rechecked the fuel system and rebuilt the carbs more than once and they are set with about 3 turns open. Have checked for leaks on the intake side and blocked off both the crancase venting take off and the choke system feed from the carb bowl. (I thought there was leakage past the choke valving system) Having rebuilt the engine recently, I did check the valve timing both with dial gauge on the followers and with timing marks. Question is - could the lumenition fail like this and cause a weak spark under load? The free spark is blue and sharp. I have a ballast coil on it.
Could this 'dropout' and intake backire be ignition or cam timing-related? I have had the car 18 years and she doesn't usually beat me so comprehensively!

Answer
Yes, the ignition could fail like this; I recently had a neighbors old deere tractor with points behave this way; OK at idle, sometimes wouldn't start, but when under a load it would miss something severe.  Healthy blue spark and new ponts, but the problem still existed.  Replacing the coil solved the problem.  I would suggest the same to start.