Now that it's finally starting to cool off, everything's working fine with the new controller. I think part of the problem was that I don't have enough cooling capacity to maintain temp AND remove the extra heat from 600 lb of iron and aluminum that heated up to 198F before the fans kicked on.

On Friday night it was a bit cooler (around 80F). The temp stayed on 180 down 2-3 miles of 30 mph dirt road until I got to the highway, then it very slowly increased to a maximum of 184 at a steady 60-65 mph. Came back down to 182 when I got into town, and back to 180 idling in the parking lot. Same thing coming home at lower temps (74F). I'm not planning to mess with it any more this year.

As I've said, the car has no a/c so I hardly ever drive it in the sticky summer heat over 90F anyway... and with the old fan switch, it never got above 198 even on a mid-90's day.