I have a "Day & Night" gas heating/air 5 ton unit. When I turn on the heat in the morning all works fine but the unit shuts down before the desired temp. It's not the thermostat because I know enough to go out to the unit and bypass the thermostat. Nothing happens. After an hour or so, the unit mysteriously will come on again. I'm not sure it helps but throwing the unit breaker panel switch off for a bit may contribute to correct operation late on. Once it comes on, it will work fine for the rest of the day. My assumption is that some electronic part heats up after being on for a relatively long period and then shuts the system down. Later, when the unit starts again, since the unit isn't on nearly as long, it cycles OK. I tested this theory out by lowering the desired temp and then gradually increasing until we reached our comfort temp (of 68 degrees). This seemed to work.

Any thoughts?

Can someone give me some suggestions so I understand a bit more about what might be going on before I call someone again?

Thank you so much.

I am sure you checked your air filter? There are safety devices that will turn a furnace off if there is some kind of a malfunction. This should be checked by a qualified service person.