Seems like amperage is like money.(there's NEVER enough).
I bought a Powermaster rated @90A and still loose a little ground @idle. Of course my load combined is 46 amps and I have an underdrive pulley. The alt has been measured @ 90 amps when the engine revs some.
In your case the under drive may be the reason
you still have a idle issue.. if you had the stock
rpm ratio it would most likely take care of it..
they still need to turn X rpms at idle to have a
positive output(that will exceed your total draw)
EDIT
I can see a fluctuation at idle if the fan kicks on
and it has a rpm drop(before the AIS kicks it back up
so that can last maybe a second
