This problem is easily overlooked. Check if the radiator is clogged with debris before breaking your head fusrther. In many cases is not easy to see: you have to remove shrouds or vanity covers, and with the help of a flashlight see how you're doing down there. Specially in farming or other dirty industrial uses, radiators get practically clogged with debris and trash and still have a reasonably clean condenser. No matter if your car has electric or mecanic fans, the problems is the same. More often, we find this condition in pickup trucks and SUV's. There are also moving vacuum cleaners, like the 93 and newer Camaro and Firebird, from which we've pulled plastic bags. So the next time you face a high pressure condition and you did everything right, check the radiator.