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Issue could be a number f things.Lets start by getting me model on units so I know what type of system we are dealing with.
It helps some. It tells me you have a conventional non heat pump system, Do you know if the furnace is gas or electric?
OK if electric the issue could be a heat sequencer somehow getting energized.
THe next time it does this, disconnect the thermostat wires at the air handler and see if it stops.If so issue is external such as thermostat if it continues to blow hot, issue is from a sequencer or something in air handler.We can go from there diagnosing.We would need a meter to troubleshoot. If you have a meter let me know .If not you can get an inexpensive one at Walmart for about 15 bucks if you would like to troubleshoot and I would be happy to guide you.