What causes overheating at 70 after driving for 45 minutes.

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100K Had an engine light and the oil looked bad. The oil pan was dropped and cleaned. Mechanic said it did not look that bad. If it runs good and oil looks good in 5k we'll do timing belt and water pump. After 4 months and 2K miles car was running fine and oil looked good. So he replaced water pump and timing belt at 102K. Within 1 week it overheated. It was while air conditioning running in stop and go traffic. In a 2- 3 mile area. In 3 visits to the mechanic who did this work as a possible solution: 1) Flushed radiator ; replaced the cluth fan 2) replaced the other fan 3) replaced thermastat. I no longer turn on the air but if the car is run on the interstate at high speed for any real length of time (30-45 minutes) it will start over heating. I turn the heater on and it sucks out the heat long enough to get home. I live and work and do 90% of my travel in a 10 mile radius. The car sounds and drives great all the other time. I have only 105K on this 2001.But some times you have to drive 70 for about 45 minutes. Do I give up and trade it in. I only put 4-5K miles a year at tops. Take it to the dealer since this tech has not figured it out.