what fuel are you running?
can you pin point a time when this began?
what has changed since the problem began?
does the engine spin normal speed on the starter?
as an experiment, use a clean/new test spark plug with a very wide gap [1/4" or more gap] begin by removing the secondary coil wire from distributor cap, insert test plug, ground test plug, spin engine, spark should be strong and bright.
If spark looks good, reconnect coil wire to distributor and do same test at a spark plug wire. In both cases the spark should be strong, bright, consistent.
This may help determine which area is the problem.
Larry Woodfin, Flyn with the Race Bird