Sounds like your mechanic is a parts changer, not somene who can diagnosis it before spending a lot on worthless parts changes. First I would look in the radiator any crud in there? Toyota red fluid is in it I hope, if not get if flushed and changed. Second go ahead and replace the radiator cap. Then inspect all hoses, tighten all clamps, replace the thermostat. Verify that both fans are working and keep an eye on the temp guage. I think 2nd fan comes on with the AC on, try that. If these dont work, maybe the water pump. Hoses on these are incredible and will last 150,000 miles or more. Use Toyota parts only not cheap junk.