Reading this thread it seems the assumption is a low speed problem but in one of your post you mentioned the new radiator seemed to run hotter at freeway speed as well.?? Basically there are 3 types of hover heating that covers most occurrences. # 1) Generally associated with age and infrequent service or maintenance where the system (water jackets in block, heads, and the radiator) just gets overwhelmed with sediment and sludge and prevents the system from doing it's job. Thermostat, t-housing, water pump, and cap are all included here for general maintenance reasons. # 2) Most common with older cars not designed to handle the kind of horse power and upgrades we tend to throw at them is low speed and idle temp increases but cools off at the higher freeway speeds as that horse power kicks in. Can't drive it on the street at low horse power or heat output but tends to cool down on the freeway when engine heat output is elevated # 3) Would be increases that will get worse at speed and never recover as you reduce speed. Obviously your car is not number 1 since everything in the system sounds near new including the radiator. Number 2 was my first opinion until you said the new radiator tends to run even hotter at higher speeds. The three things going on in the cooling system is air flow through the radiator either created by highway speed or actual MPH of the car or the fan on the water pump (or electric fan), the coolant flow through the block, heads, and ultimately the radiator, all of which is controlled by the water pump and thermostat, and temperature removal from the system by the radiator. At high speed, overheating when the air flow is at it's best and coolant flow determined by the water pump and engine RPM's is at it's best the three things you look at first is water pump, thermostat. and radiator cooling capacity. The most expensive being the radiator I would replace the thermostat first and if fuel economy isn't the biggest issue for you lower the opening to 180S or even 160S. The "S" (super) usually indicates a slightly larger plunger opening that shouldn't exceed 10-15% increased flow rate that helps at low speed and idle and doesn't create a problem at high speed. Water pump is next from a cost issue and last but not least would be the radiator. If the new radiator doesn't cool as well as the original unit at high speed when all else is doing it's job and your pretty sure the engine is good (not over bored and assembled properly so not to cover water jacket flow) you can pretty much be suspect of the cooling capacity of that new radiator. There's a lot of $200 aluminum radiators out there and most, if not all, are made off-shore where labor is cheap and engineered for everyday low horse power use. Once you solve the high speed issue when your car is making heat you can better understand and solve #2 which is usually air flow and coolant flow issues and not radiator related. I'm guessing 60% of low speed/idle overheat issues where the car cools down at higher speed is really air flow issues where the engine compartment, pavement, air box design forward of the radiator, or shroud missing is the only thing that needs to be upgraded as well.
