Overheat gauge

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When I put A/C-Heat selector in "off" or "Max A/C" position, within a few minutes of running, the overheat gauge and light come on, indicating overheating. It is not overheating. Has been doing this since new. Original dealer (in Fla) removed thermostat to solve problem (unbeknownst to me). When I moved to colder climate, 3 yrs later I discovered thermo was missing and replaced...since then, problem is back. Two dealers have not found problem.