2000 chevy over heating?

i live in the bahamas and we have been having temps like 95 degrees, so when i run the blazer's

a/c for long periods of time during the day in stop and go traffic the temp gauge goes beyond the mid point. if the blazer is running with a/c and the is no stop and go traffic the gauge stays at the mid point, but if i turn it off and back on i see that the gauge goes beyond the mid point. but if i do not have the a/c on the gauge never goes beyond the quarter make. if i run the a/c and i see the gauge going beyond the mid point i turn the a/c off and the temp returns to normal. sometimes the resivoir bubbles and becomes empty along with the radiator. but this doesnt happen all the time only when it is an extremly hot day outside. do you think if i hooked up an auxilary fan to assist the clutch fan that this will provide proper coolinfg from the fan. any suggestions