A lower temp fan switch will not cure your problem, been there and tried it...
You need to increase airflow to your rad, at 65mph theres much more wind going into the front of the car than a pair of fans can ever reproduce, so if its not cooling then the fans wont cool it either.
Try running it with a standard ic on for a few journeys, if its fine then youve got an airflow problem. I swapped my IC and ive driled some holes in the bumper to aid airflow.
A lower temp opening stat is a good idea, ive got an 82c stat.