it is possible there are plugs not firing and loading up / fouling :ambivalence:
what plugs do you have in there??, new-not just regapped used ones??:hopelessness:
try 8-fresh regular plugs gapped at 0.030-0.035''
it did sound like you needed more initial timing [14*-22*]also to let it run cleaner,
just limit maximum total advance [all in by 2500-rpm] to 34*-36* in the distributor:saturn: