that is a really, really bizarre problem.
For something so strange as that I would start to look at my earth connection, but after a quick check of those I would then go through all of the multiplugs between the dash and the front of the engine. There are roughly 3 that join the in car loom to the front of the engine looms - and they can cause problems such as you describe.
Open up the multiplugs and see if any of the pins have discoloured / look like they have over heated. I have seen many times, on many cars wires bridging around multiplugs to overcome the problems which may be similar to what you are experiencing.
Worth a try....
Also, just check that the strips of cable harness are fully pushed in to the back of the fuse board. There about 10 or so individual strips of cables on plug strips, that plug into the back of the main fuse board, check that these are fully bedded in place.
Check for a good live and neutral connection at the battery.
Check for a good connection of all of the earth where there collect together on the engine bay crosswalk.
Finally, remove the cowl around the steering wheel and check that the 6 multiplugs that clip in around the steering column, that interface with all of the switches are all correctly bedded.
Oh, one final thing, open your front doors and slowly move them back and forth whilst operating the window motor switches. I have seen before very small breaks in the cable allowing some current to flow to earth, which can upset all sorts of things.
I do however think that from what you have described above - you have a bad earth.....