I gave my son my 1998 Tacoma with about 250,000 miles. I bought it new and it has had regular service since new. Recently it started overheating at highway speed 70 mph after about 1/2 hour. It does not overheat if he drives slowly. He had the radiator flushed and tried taking out the thermostat but no luck. I would really appreciate any ideas you may have. Thank you very much. Mark