After you re-grind the tungsten, use a Red ScotchBrite pad and wipe the tungsten. It cleans and polishes it up nicely (will even remove the discoloration). Just wipe from middle to tip. And turn up your gas CFH and allow a longer post-flow. It helps protect the tungsten, and loitering over your weld with Argon only helps keep your puddle clean.
Preventing contaminating your tungsten can be "improved" using a few tricks:
1 - Rest your arm/hand holding the torch and slide it along. This eliminates you from "air welding" and moving closer and farther from the puddle as you move along.
2 - Rest your cup directly on what you're welding, tipping/tilting it forward to point the tungsten at your rod/puddle. Then just slide the cup along the metal. This virtually eliminates any distance variance, and allows you to get and maintain a very tight arc.
3 - As you dab, adopt a technique of: add, slide, hold, add, slide, hold, add, slide, hold...
What this means is once you have your puddle established, dip the rod, then slide the cup, hold the cup still, add some rod, then slide again. This helps you dip the same amount of rod each time, and move the puddle/torch without introducing wire and making too large a puddle.
Using a #7 or #8 alumina cup with a gas lens helps immensely with your welding and arc distance. The stubby #8 cup is a wide body and enables you to rely on it as a resting surface and positioning prop. All that, and a good Flexible head torch! That is the other really advatageous trick.