Given that you performed quite a number of checks, here are my 2 cents
First of all if you are feeding your antenna(s) with coax, ensure to add some good chokes along the coax to take care of CMC noise, a good design is winding about 17/18 turns of RG174 over an FT240-43 toroid core, the reason is explained here
http://www.karinya.net/g3txq/chokes/ensure to add at least two chokes to your coax, one near the antenna feedpoint and a second one where the coax enters your building (add more chokes if you can, they won't hurt)
If, after choking the power lines and the feedline you still face the issue, do NOT remove the chokes, since they'll still be useful, instead, consider the idea of using a separate RX only antenna; a good one, which in my experience will help A LOT is the one described here
http://www.kk5jy.net/LoG/I know, it may seem crazy, but believe me, it works; just ensure to build it "by the book" (feeding transformer, wire len and so on) just, to stay on the safe side, add a couple of chokes to its feeder, that antenna WILL make a difference