1990 Dodge Truck hot running at highway speed

my 1990 Dodge P/U has been running hot at highway speed for about 8 yrs. I have replaced everything that I know of the cooling system, radiator, hoses, thermostat, temp sensor, water pump, and fan clutch, all to no avail. Once the ambient temp reaches about 80 degr. it runs hot. I don't think a bad head gasket would last that long. there is no water in oil or vise-versa. Just had the cat and muffler replaced, half of cat was in muffler. Any ideas??? I've asked 4 different mechanics and no one seems to have any other ideas.

Thanks in advance. RAH