I suspect that it is caused by the dirty big front mount intercooler that I am currently using. The dimensions of the core are. 480mm wide x 300mm high x 70mm deep. I'm thinking that not only is the intercooler restricting the air flow to the radiator, it is also heating the air during the exchange process and therefore the air that the radiator does get is hotter than it could be.

I'm tempted to try a smaller intercooler, such as one from an SE. I'm not planning on trying to extract huge amounts of power from my turbo setup and therefore don't need such a big intercooler. Before I do this, I was wondering if anyone else here has any thoughts on this matter?
J