Electric fan won't cool at highway speeds

I would add that it is possible the cooling passages in the block are full up to the freeze plugs with Scale...and coolant flow is taxed as well....

I bet there is an undersized or clogged radiator behind all this.

That could have been a possibility if he didnt state that this problem didnt exist until he swapped to the electric fan and shroud. High speed cooling was not an issue previously, which suggests his system is capable of cooling his engine once this is sorted out.