I have central air in my house and the unit and everything is about 20
y/o. I realize it should be updated but I have no control over that.

The problem I have is that if I set the thermostat to to 70 degrees
fahrenheit, then the downstairs stays about 70. But the upstairs will
be anywhere from 76-80, depending on the room and how hot it is
outside. So the problem is that people complain that it's too cold
downstairs and too hot upstairs. Ideally, I would like it to be about
76-78 downstairs and about 74 upstairs.

Why does this happen and is it normal? I know that a lot of air blows
out in the vents downstairs and not a lot blows out upstairs. I shut
off all the vents that aren't needed and close the doors to rooms that
aren't used. I have the fan set to "auto". I don't know if leaving it
on auto is better than setting it to "on". I don't like to mess with
it because it seems to cause problems with it.

Any opinions or suggestions (other than buy a new unit)?