Brad C wrote:

snip

The high temp in the attic is definitely causing cooling loss during the day, but the thing that concerns me is during
the morning, when I took the last set of measurements, the attic was only 74 degrees. The supply was 64 and the return
was 69, so the unit was dropping the temp 5 degrees with an attic temp of 74. When the attic temp was 110, it was
dropping the temp 7-8 degrees. This would seem to indicate that the unit can't cool air beyond 64 degrees. That seems
like a problem attributed to something other than cooling loss thru the ducts since the unit on the 1st floor drops it
17 degrees under pretty much the same conditions. The difference between the two units is upstairs is 2.5 ton and
downstairs is a 3 ton. That shouldn't cause the problem I'm seeing.

snip

I don't know if this is your problem or not, but you are forgetting a
major part of your equation. The condenser. If the condenser cannot
remove the heat that's being removed from the house, the system will not
work as efficiently. What shape is this particular condenser in?
--Mike