Our house is two story townhouse.  Furnace is in attic, Ac compressor is outside by house like usual.
Furnace is 20 years old.  AC was installed later about 14 years ago.
A few weeks ago the AC stopped working.  Even the house fan wouldn’t work without AC.
Repair guy came out and at first thought it was because we were low on refrig.  So 3 lbs later and $900 he says we should be good.
Definitely way cooler after the recharge.
Ran fine a day or two then out again.  We didn’t run it at all for 2 days since we were out of town.
He came back and it turned on for him.
He said maybe it was a capacitor, so he replaced for $100.
We were good for 12 hours and out again.
He came back this time while it was dead and changed our thermostat.
The temporary thermostat he put in didn’t have battery back up like ours did so he found out when the problem was happening there was no power being sent to the thermostat.  He goes back to attic resets something and power restored.  
Went fine for a few days with temporary thermostat.
Today it did it again while I was at work.
Wife called them and he says he thinks it is the furnace mother board.  
This will be another $400 installed and will take about 4 days to get the part.
When I came home sure enough, dead, no power to thermostat at all.
I check fuse box and it was fine.  i did cycle off and back on the circuit breaker.
Still nothing.
Then just randomly 3 hours later it just turns back on.
Still running now.

What do you think?  
Would a bad mother board make it lose power and latter power come back on?
Sound correct?
Another other ideas?

We know we will need to replace the furnace and ac eventually as the are 20 and 14 years old but we are hoping to wait a few more years before dropping $8-$10K to do both.

My first thought was check tstat, then circuit board.

Lack of freon does not make the indoor fan stop working.

Your tech isn't a tech, he's just guessing.

Sounds like it probably is the board, they can do weird things on the way out, but I'd still find someone else unless this guy is giving a credit on all his other guesses.

Technically, lack of refrigerant doesn’t make your fan stop working, but it does cause a frozen coil which can make it seem like the fan isn’t working.

If you have an upflow, once the coil thaws some of that water will end up in your furnace and can cause electrical issues.

But being in an attic, I would think you have a horizontal configuration. Less likely for that to happen in that situation.

He is using the parts shotgun approach, find someone else.

I had mine lose power to the thermostate due to a lightning strike.
I figured out where the power loss came from on the board and wired a jumper wire between those 2 spots and the a/c has been working for the last 12 years like that.
I finally got a new board, just too lazy to put it up there knowing what it is.

My first advice is to find a new company to assess your problem.

This, you don't have a tech, you have a parts cannon! New company, now.