The "weak link" in your setup for aluminum is your WP9 torch. It's only rated 100 amps AC @ 60% duty cycle. (Based on: http://www.weldcraft.com/products/wp-series/wp-9/ )

Although it might be able to handle welding short stitches of 1/8" aluminum, welding very much of it at a time risks damaging your torch and/or cable.

As others have mentioned, you can take as much heat as possible off the tungsten while in AC welding mode by rotating your machine's AC Balance knob fully counter clockwise (which on the 200DX as I understand, should put you at 30% EP / 70% EN).