To efficively cool any space, whether the garage or attic, you need air
intak vents and air outlet vents. As Mac suggested, I would check
where the air is coming in. It's very common for insulation to be
blocking soffit vents. There are plastic baffles that are made that
you can staple to the rafters to keep the channel open. I'd also check
to see how big the soffit vents are or if they are blocked with paint.
You can install new or bigger ones.

For the outlet, I think a continuous ridge vent is the best way to go.
Doesn't use any power and provides even ventilation along the roof.

Adding additional insulation may help too. As for the A/C problem,
it's possible the system was designed poorly from the start. I've seen
lots of systems where there is inadequate ducting to the upstairs far
end reaches. An add on blower may help some, but may not solve the
problem entirely. Gettting the attic ventilated will help. Also check
the returns for upstairs. Sometimes they are undersized and enlarging
the grill opening will help. To really solve it may require either
installing more ducts or possibly a second ac for the upstairs.