Steve Kulpa wrote:
Don't know about any insulation between the floor of the bonus room
and the garage ceiling as I have no access there. I did notice a
slight improvment in the bonus room when I cooled the garage down, so
maybe there's none? I DO know there's plenty of insulation
everywhere else as I can see it from inside the attic.

Sounds like twstanley put his finger on it; the most likely source of
the problem is heat gain from the garage, most likely due to the lack of
adequate flooring insulation in the bonus room. Heat gain from the
attic through insulated (R-6 or better) ductwork isn't significant. I
doubt that power venting the attic will do anything to change the
situation.