The head redesign has NOTHING to do with the cooling system. It was a flawed casting, they adjusted it several times. End of story.
About the cooling problems, David, I think you have something wrong with your car. I would strongly suspect the radiator, assuming your thermostat is OE and fairly new, and you have a good radiator cap. Someone else had similar problems recently with an 18 month old radiator, and installing a new one cured it. Don't think that a 3-year old radiator is "good". I assumed that when I bought my car in 1997 with a 4-year old radiator and I was seeing temps of 110-115C in the summer. A new radiator cured that.
FWIW, after I replaced the radiator, I could climb I-80 to Reno up steep grades with my foot to the floor (near full power) with the A/C on in approx 95F ambients and my temp would NOT go over 100°C. I think that kind of poo-poo's the theory you guys are coming up with about inadequate cooling capacity. When new, or functioning as new, there is NOT a problem with the 124/603 cooling. However they seem to be very prone to plugging up the radiators internally (the core turbulator fins probably catch more crud than they improve cooling
) and that's my theory why the radiators may need replacement.
Note in this
factory PDF document that "high temps" are considered to be over 110°C, and that the solution (after ruling out other causes) is to do a citric acid flush with the OLD radiator in place (to catch the crud), then replace the radiator!