My only other suggestion for your "popping" sound, and Rich-The-Jackson-Rep will be able to answer this a little better, is: are you sure you've got good quality tungsten? I recently picked up a couple of sticks of 2% Thoriated from my LWS that wasn't in very good shape....have you swapped out tungsten or considered going back to your LWS and getting some new stuff?
Also...how are you grinding your tungsten (sharpening it)? Is it possible that you're contaminating it on your grinding wheel?