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I noticed a strange thing happening on my upstairs HVAC unit a few weeks ago...

The thermostat (digital) was in the COOL mode and set to 72°.  It was a cool day and the room (per the thermostat) was about 67°.  I happened to be upstairs when the circulating fan came on.  I checked the outside compressor unit and it was not running.  I then set the mode to OFF and the fan stopped after about a minute.  I set the thermostat to HEAT mode and set it to 68° and all seemed OK until a day or two ago when the circulating fan came on when the room was about 70°.  Any ideas why this would be happening?

You may have a short in the low voltage thermostat wiring, bad thermostat or faulty fan control in the furnace.  Sometimes the fan runs if a safety trips, but unlikely if your system was off.

Is this the same unit that has been giving you heating problems?

Some thermostats have a fan on, auto and circ/prog mode.  Make sure you have it set to auto.  The Circ/Prog mode cycles the fan every so often.

What kind of furnace and thermostat do you have?

This is not the same system that is giving me the heating problems though is is the same brand (Goodman).

It is a Robertshaw digital thermostat (not programmable).  It has 2 buttons: 1 for setpoint up and 1 for setpont down.  It also has a 3 position slider for Heat - Off - Cool and a 2 position slider for Fan Auto - On.

If I put the 3 position slider to Off and the 2 position slider to On, the fan will run.  When I put the 2 position slider back to Auto with the 3 position slider still in Off, the fan will run for about 60 seconds longer then stop.

I don't know if this had anything to do with it but I changed the batteries in the thermostat and I haven't noticed it happening since.  It seems unlikely that old batteries would cause the fan to come on though.

It still sounds like a bad control board.  Every post you've made leads me to think you have a faulty control board.  Eventually it will fail completely.