You don't want to hear this and you don't want to think about this, but. If you do a search of the posts on this sight you will notice that there are more than a few that start like this one and end with a leaking head gasket or for these 603 motors cracked head endings. Some believe that the higher temps occur when the engine is running hard because hot combustion products can get forced into the cooling system increasing temps, some believe that gases replacing coolant in the head spaces allow localized overheating or at least localized temperature disparities that contribute to head cracking. I've got an 87 SDL that has been temp freaking, that I'm about to pull the head on so I can check the gasket and for cracks. These heads and the gaskets have been redesigned numerous times each. It may be coincidental but I started to notice my car's symptoms soon after I rehabbed my turbo boost system. The boost control pipe from the manifold was plugged with carbon, I cleaned it and the restored boost was working better than ever, I wonder if the renewed increase turbo function stressed the engine causing a head gasket leak/failure? I'll know something when I pull the head this week.
P. S. Why do you think your radiator is "plugged", the increased temp? Could be at least a couple other cheaper things, no?
And whats up with the distilled water being corrosive? I'm not questioning if that's true, I'm wondering actually the how and why of that, I don't understand.