LenS wrote:
We have experimented with turning off the fan after the air has
dropped to the set temperature. In every case, the indicated
temperature climbed again. Turning the fan back on again results in a
temperature drop. Not very scientific, but it did convince us.

You need more attic insulation. Numerous studies have shown that attics
that are adquately insulated (R30 - R40 in most climates) don't benefit
from powered attic ventilation.