Joel,
I have run into similar difficulty with Semi-auto GTAW on knife edge airseals..
MC grade wire is a good start.. also an improved feeder tip may be a possible solution.
You could give my friend Gregg at Percision Welding Technologies a message about your problem.. He is top in the world with semi-auto GTAW and Micro plasma..
http://www.pwt-online.com/Tell him Larry B. Says Hi.