Intermittent heat

Heat works fine most of the time but will suddenly start blowing cold air. This may last anywhere from two to fifteen minutes. It will shut it down and have to check it every minute or so. Had a mechanic check out thermostat, radiator, heater core, says all checks out. Friend said he had similar problem with a car and it ended up being an A/C switch, or something to that effect. Has the older slide style settings. Thanks for any assistance.