Did the drop in milage coincide with the plug change?
You mention the spark plugs are new.
This required removing the plug wire connectors. Perhaps this is a plug wire resistor problem.
There are posts here on the forum which indicate the resistors by themselves can be changed.
I have not tried this yet, as until recently I thought the plug wires were ready for social security at around 130k mi.
You might also do a search on plugs, I seem to recall a suggestion that the plugs be gapped a little wider than spec, say to .035.