Just reading the pass few post and I think you should look at the "air flow" first.
Formula 1 teams found at speed a scoop creates cavitation around the edges and slow down or at least restricts the air flow.
I think at 70MPH the air is not all passing through but getting trapped and being buffeted around the engine bay keeping the heat in and over a period of time building up and blocking or slowing down the flow of external cold air.
I have known of a simple mess grill causing similar cavitation problems.
Are you using inner wings? If you are not try fitting them, if you are try removing them or cutting some vents in them.
As Scottie22 said higher speed should mean greater air flow and cooler running.
Hope this makes sense
