You may want to check out this link:
It'* a previous post about a full time fan switch. It is sorta along the same lines. It does sound a lot like an issue with the resistors to me. If you are checking power to the resistors, you can ground it to the frame. The power wire is full power all the time and only kicks the fans on when it is grounded. Also, the high speed fans kick on about 220 degrees. I don't have any answers for you, but I got tired of my car running hot and installed the full time fans. I haven't had a problems since. Oh, one more thing. Before I installed the fan switch, unless I had the ECC running the fans would not turn on until 220 degrees. Opposite the problem you were having. No answers here, just observations.