The bane of all electricians is the intermittent problem.
Back 3/4 of the house? That's a lot of circuit runs. Right? If they're home runs to the panel, maybe you've got a loose feeder, but that's not too likely, unless you have a subpanel controlling that section. Up this way, we had a lot of homes wired with aluminum wiring (plus a lot of mobiles) back in the 70's and of course now, the problems are like an avalanche, the heating and cooling of wire is loosening the connections and the cheapo stab backs don't help either.
The only other thing that comes to mind is a buried J Box. I had problems with a water heater until I did a remodel and in demo-ing a wall, I found a buried j box, no access of course. Yep, it fed the water heater and you gotta love wire that was twisted together, then to protect it, wrapped in tape. No clamp, no wire nut, just twisted. My house was built in 1903 so I guess I should have expected lots of DIY remodeling before I came along. How old is your house? Just something for you to think about.
