Something must be amiss.... if the fan is coming on (at the right time???) then the car should not overheat.
It could be a simple problem, like the radiator cap loosing pressure, which will then allow the coolant to boil at a lower temperature.
In fact anything that doesn't allow the system to hold the correct pressure "over" the coolant, will make it boil.
ie. a leaking water pump, a leaking head gasket or even something very simple like air trapped in the system.
In 2000 spyders the highest point of the engine is actually higher than the radiator, so they can be difficult to get all the air out.
The solution is to install something like a "flushing T" in the heater hose that comes from the rear of the cylinder head, allowing you to bleed the air out, and top the coolant right up, and excluding any air from the system.
No "magic" additive will fix the problem, pure water is one of the best conductors of heat you can get....
SteveC
