1. at highway speed (anything above 35 unless it's a truck) the fan is completely unnecessary, and an electric with a shroud may be restricting air flow.
2. can any air enter the nose of the car without going through the radiator core (bad hood-to-cowl and rad support seals)? Fix that.
3. let me guess: no vacuum spark?
4. add power valve, or you're liquid cooling with rich mixture.
5. core face area may be too small, you need about 1 square inch per HP, core thickness is nice but it doesn't have the same value.
6. if this is auto with the ATF cooler in the rad, you need an aux ATF cooler.