If not done yet try:

Prestone (or any good brand) cooling system FLUSH
Use 50/50 premix coolant ALWAYS (it's made with distilled water & the correct 50/50 mixture cannot ever be screwed up) unless you use..?
Check ignition timing too advanced will cause overheating
Hold a piece of stiff cardboard against the fan blade with engine at idle ~ if the blade can be stopped then the fan clutch is NO GOOD
Try to avoid aluminum & plastic radiators

Good Luck