Don't know if this will help, but I had a similar problem.
Earlier this year, my efficiency dropped from 80% to around 67%, which coincided with a perceptible difference in the taste of my brewing (filtered tap) water. Turns out the pH was around 7.8!
My last brew, I adjusted the brewing water with food grade 10% phosphoric acid (from LHBS) to approximately 6.0. Mash pH was 5.3, and efficiency rose to 76%.
I never used to measure pH before, but then the water always tasted the same. In another state, I used artisinal wellwater to brew- it tasted great, I regularly got around 83%, and I always got fantastic creamy foam with good retention. Sure wish I could fing good water around here.