20 year old GMC 5 ton unit. Manual fan switch turns fan on, but in auto position the fan intermittantly will not come on, or shut off. I am operating the AC unit with the fan switch on all the time.

Sounds like a relay switch plumb wore out and needs replaced.

I would suspect the thermostat, but it somewhat depends on the thermostat design. In AC mode the thermostat controls the fan.

So, the tstat must have a relay to turn on the fan.
If the tstat is designed such that it activates that relay in AC mode, then it doesn't make sense.

But if there is a real physical switch that bridges the relay contacts, then the diagnosis makes sense.

Checking, depends on your skils and if you own a multimeter. Do you?

20 year old GMC 5 ton unit. Manual fan switch turns fan on, but in auto postion the fan intermittantly will not come on, or shut off. I am operating the AC unit with the fan switch on all the time.
The tstat is Robertshaw; but with the old mercury switch tstat, I sometimes would have to manually cycle the switch to get the blower to turn off.
Now when the tstat is in auto, the compressor and condenser fan will come on, but the blower will operate intermittantly; sometimes it will come on or go off with the unit, sometimes it will not. I disassembled the relay, it looks fine inside. The relay is energized with 24vac manually through the tstat switch, or automatically from some other switch or relay; I haven't been able to locate that source to check for 24vac.

Look here: Thermostat wiring code (http://xtronics.com/reference/thermostat_wires.html)

You need to look at the voltage between R and G when the system screws up.
It wlll be 24 VAC if it screwed up and near zero volts if the relay in the stat is closed.

There could be 3 things that control the fan:
1) A switch on the tstat (not always)
2) A relay in the tstat
3) A relay in the furnace

I'm suspecting the stat relay.