your comment abt iridium plug is interesting.

I decided to change my original plug @ 8.5k miles (instead of 12k interval) because of the engine vibration had become too noticeable.

the change got the engine to purr again. Didn't go for iridium as I wasn't sure if the plug change would cure the vibes.

At the next change I will try the iridium if the fuel consumption improves