Brand X is right!
Diamond Ground has a guidebook which you can download or print off that is about the most accurate guide around.
http://www.diamondground.com/howto.html
If your torch is water cooled than try 1/16 (0.063) Zirconium, it still has the best low amperage AC arc performance, especially with a pointed tip. Remember to avoid the long slim tapers and go with a more radical angle. Thorium is ok as long as you don't overheat it. Thorium will not form a ball when it gets hot, rather it cracks or sprays into your weld metal. Cerium the #2 choice and best if your torch is air cooled. If you have pure tungsten (green) toss them.
I checked out the owners manual for the TSW185 http://www.thermadyne.com/tai/literature/pdfs/service/430429_503.pdf (check page 24) and if yours has similar features than you should increase your AC frequency when arc wander becomes a problem.. Normal AC runs at 60 Hz. Try increasing your AC frequency to someplace between 120 and 200 Hz. This should have the effect of "stiffening" your arc and making it more directional.