Are you using the high helix two flute cutters from GARR? I think the code is 242M, they have a 45degree helix. We machine a T0 aluminum alloy and find these are the best cutter for clearing the chip out.
I had described our conditions in this thread:
http://www.cnczone.com/forums/showthread.php?t=60698 (TM3P Performance)
I would also use a larger diameter cutter if possible, something like 15mm to give more room for chips.