10 ga is great!

Not to be too elementary, but did you check to see if the fan was moving air the correct direction? Sometimes, with the shroud, the exiting air can fool you. Take some paper (like a $100 bill) and put it in front of the radiator to see if it is pulled against the radiator, indicating the air flow is front-to-rear. If the thing were reversed, it would cool at idle but stall the fan at speed.

Such a problem is common with belt-drive fans where someone uses a v-belt fan on a serpentine system or vice-versa.