The issue is that when the engine is hot enough it will boil the coolant and can overwhelm the coolant overflow. Once that has dumped coolant out it will eventually cool and the vacuum of the system will draw what coolant remains back in. The trouble is after a few cycles of this it will draw in air and that is where issues begin. Advice hear is to use a slightly higher percentage of water in the cooling system as water transports heat better than coolant. Next to keep the water from boiling, or expanding as much when it does boil- use Redline WaterWetter. The stuff works very well and I've been using it for years. Finally consider a turbo timer if you are too lazy to let the car idle when shutting it down.
I've been dealing with similar troubles on my ST165 and feel I have a fairly good solution.
viewtopic.php?f=44&t=43563&start=60#p421897 If you take a close look at the lower and upper grille openings, you will see that there are many areas for air to go around the radiator as opposed to passing through it. Nor is there much to keep the air from washing through the sides of the radiator. Note that air will flow from high pressure to low pressure and that there is a large high pressure zone in front of the car. Additionally, with the high pressure zone in front of the radiator we need to ensure that the pressure is lower behind it so that air will pass through efficiently. So there are a few major hurtles to overcome.
My solution addressed airflow to the radiator and the pressure zones on either side of it. Firstly, the aluminum panels that closely fit to the bumper support and core support. These surround the radiator and prevent air from washing over the top and sides of the unit. For air passing underneath there is the lower 'scoop' that mates with the bumper grille and directs air to the radiator. Turns out I never actually put a picture of the finished product but you can get the basic idea. As for the pressure zones, the splitter does a few things here.
viewtopic.php?f=44&t=43563&start=75#p422151 First it creates a high pressure zone above the panel and directs air to the vents above it. Next it keeps the air that passes below it smooth and laminar. This creates a lower pressure zone behind the panel, which helps to suction air out of the engine bay. This effect is greater with speed, and to address lower speeds is where the vents come into action. Heat naturally rises and the vents help bleed off extra heat from the engine bay decently at lower speeds.
Oh- nearly forgot to mention- get some foam weather stripping to seal the entire radiator to the core support. There are many large gaps that allow air to escape around the radiator. This effectively makes the pusher fan in front of the condenser useless unless the air spillways are addressed.
In either case I hope this gives you some insights to the cooling system and areas that it needs improvement.