2002 Toyota Tacoma Engine overheats (according to temperatu

My engine started heating up (according to the temperature gauge) recently. The temperature starts rising after I have been traveling at freeway speeds for about 20 minutes or so. It seems like if the weather outside is cooler it takes a little longer to heat up. I had the radiator replaced with a brand new OEM radiator last year, and since the heating problems began my mechanic has replaced the thermostat, sender, radiator cap, and water pump. When he replaced the water pump he pointed out that it was leaking, and the part of the pump that spins had been rubbing inside the pump. When I saw that, I hoped the problem would be solved, but my truck still heats up. When I run the heater the temperature goes down. When I turn the heater off the temperature starts rising again quickly. Also, I took the truck in when it was running hot and my mechanic measured the temperature of the engine on the "hotter side" at 212 degrees, and the "cooler" side was maybe 180 degrees? I can't remember the exact number. According to the mechanic those readings were pretty normal. I'm hoping someone can help me out with a possible solution.