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Air conditioning/heating/ventilation system

on occasion hot air would blow out from the vents when the a/c is on; I would either turn the fan off/on and/or the a/c control button and the problem would go away. Also: one cool evening I had the fan on with circulation from the outside air coming in. Amazingly, after a few momutes very hot air started coming out from the vents. This leads me to believe the problem is not necessarily with the a/c system, but elsewhere. Any ideas? Thanks.

Did you have defrost on? I would start by looking for a vacuum problem, unless your Highlander uses cable controls for that system.