[grego37]

There are many factors that involve cooling.
Air/ fuel mixture
Radiator
old thermostat
old radiator cap
Fan problems
head gasket problems
block bored too far
etc...

If you've checked all the basics, go to the highest rated radiator cap your system will allow. You can actually raise the boiling point of your water by doing so.

if you live in an area where freezing isn't a concern, plain water is the best for cooling. You will need to add a rust and corrosion preventive (redline water wetter, prestone, there are others too)

I would highly reccomend going to stewartcomponents.com
Guru's in the cooling field.
They offer:
blueprinted water pumps
blueprinted thermostats
high pressure radiator caps
etc...

and tons of proven knowledge regarding your vehicles cooling system. They even have a question/ answer board.
good luck