Thanks to all for the suggestions re my quandry. The actual one that worked was changing the spark plugs from Champian X A25's to Motorcraft F11. The car is a different animal. I didn't think the change would be so dramatic. The Champian plugs by the way were brand new so I was always assuming that they couldn't possibly be at fault. I now know what it feels like to stop hitting your head against a brick wall. Thanks to Bob Coiro especially.
John and/or Bob,
Why the big change using the Motorcraft F11 plugs? I'm running new Champion A25 plugs. No problems so far, but I'm curious about the improvement with the F11 plugs (?)