Why is my slanty chugging? (Problem solved, for now anyway)

Okay, I just replaced all six plugs with new NGK ZFR-5N plugs, and the problem went away.This car does seem to go through a lot of spark plugs. My maintenance records that came with the car (going back to 1973) reveal the same thing. Lots of spark plug changes. Anyhow, thanks to all who weighed in on the problem - it's much appreciated!