I had the same problem last fall and everyone said ignition switch. I kept troubleshooting and was convinced that it was the fan switch. I replaced it and the problem went away. How I nailed it down was, once it got to where the problem became constant instead of intermittent, I disconnected the harness from the fan switch and it went away. I turned the switch to off and plugged it back in. The problem was still gone. When I turned the fan switch on, problem came back. Unplugged switch and the problem went away. Lather, rinse, repeat.
