How far of a gap is there between your intercooler and yor radiator? the closer the two are the better your radiator will cool. Did you have any kind of ground strap hooked to the radiator as this would cause electrolysis.

 

i had the same problem with my boosted b16 hx coupe before. I had a race koyo with one of those cheapie slim fans and the cooling system like rad cap, thero switch theromstat was working, but its just that the fan wasn't pushing enough CFM to cool down the Koyo rad.

So i think it might have been the problem in your case too.

Buy the flexalite slim fan when running it with a race koyo rad.