Since it's a cruising speed I'm guessing the electric fan is inhibiting airflow across the radiator. Most people have the opposite problem of overheating at stop lights and what not. Are you sure the fan is pushing the air in the right direction? If it's not it might be fighting against the wind when you are cruising. Did you set it up as a pusher or puller fan? Did you remove the old fan as well or leave it in place?

Also from what I've read 50/50 anti freeze is good for cold temps, but antifreeze does nothing for cooling. 100% must be like syrup. Water cools better and anti freeze raises the boiling point. In the ponti manual it says at 15psi the boiling pt of water is raised from 212oF to 258oF. According to Boyles Law of ...just kidding, I think that's for a gas anyways. I just re did my water pump and I'm going with 20%anti freeze 80% distilled since I'm in Cali and it never freezes where i live. I've heard you can go 100% distilled if you add an anti corossion solution.

Some good info here:
http://www.flowkoolerwaterpumps.com/cooling_faq.html
"#7. Coolant composition. A 50/50 mixture does not help cooling it raises the boiling point. Run 100% distilled water with water pump lubricant or distilled water with about 15-20 % antifreeze. Both cool better than 50/50 and still lubricate the water pump to prevent corrosion in system. When the weather cools return to 50/50."