Toyota Repair: toyota v6 running hot, cool in town, toyota v6

Question
I have an 89 toyota 4x4 with a v6. The truck runs hot when I am on the highway but cools down when I am just in stop and go traffic. I have replaced the radiator,fan clutch and belts. I have not yet replaced the thermostat as I did not think that it would be the cause, as the truck runs cool in town. Any ideas? Thanks

Answer
These engines had lots of headgasket problems, so many that toyota had issued a special service campaign, have the headgaskets checked for an internal leak, and if there is a problem have the dealer run the vin# through the computer if the recall was ever done on your truck, it's worth a try.