What is the condition of the radiator are any of the fins in the core loose / corroded? Have you tried back washing the outside of it by taking some purple power and spraying it through the front of the radiator and then using the hose to blast all the crud out from the back?
How long have you owned the truck and how often have you changed the coolant? When you look down the cap does the inside look like its full of deposits or scales? Does it run with a slow or fast idle? Do you know what the timing is set at?
My site has much more traffic for these problems, you can try posting there and maybe someone else has had a similar problem.
Mazdatruckin.com
Kyrasis6
Owner of Mazdatruckin.com