Hey 1200,
just a thought here, but a radiator needs airflow to exchange the heat in the tubes and fins to the atmosphere. I don't know the details about your intercooler but it would seem that when you are mobile there is enough air forced through the intercooler (which warms it slightly) to draw heat from your radiator. When stopped, all components heat up (including the intercoler) hence the radiator is only being fed a reduced amount of air which is being pre heated by the intercooler. Also, the fan(s) you are running may be struggling to pull enough air through both restrictions (ie I/C and rad.) If your shroud is not ideal then air drawn in will follow the path of least resistance - around the rad. rather than through! The long and short of it is that the cooling system (as with a turbo) needs airflow (preferably cool!) to operate efficiently. Have seen this problem more than once when people obstruct air paths with banks of driving lights or very fine mesh screens etc. Suddenly the cooling system cannot overcome the reduced surface area or static pressure.
Hope this gives some food for thought !
Cheers