Several questions:
1. As you travel across the tube, are you changing the angle of your torch. The more you angle your torch, the more you loose control of you heat. Think of the tungsten as a cone which is producing a cone in the arc. The more you angle your torch, the wider the cone becomes thus, you loose control of your heat.
2. As you travel across the tube, are you pulling your tungsten away from the tube? Increasing the tungsten to work piece distance will also cause you to loose control of your heat.
3. Is your tungsten contaminated?
Any contamination on the tungsten causes the arc to jump off the tungsten above the tip of the tungsten which is where you want all your current traveling from. If it is jumping off above or before the sharp point of the tungsten, again, you are loosing control of your heat.
When instructing GTAW, these are the biggest problems that hinder a person new to the process. Try to think of the tip of your tungsten as the driving force where you want all your heat or current coming from. When you sharpen your tungsten, are you sharpening it parallel to the grinding wheel, or holding it at an angle with the tungsten making a groove in the wheel? This is the proper process, except move the tungsten on the wheel and do not cut grooves as this severly limits the life of your wheel. Rolling the tungsten on the rest of the grinder causes your lines to go circumfriential with the tungsten. This will also cause the arc to jump off the tungsten before the sharpend tip. Making the grinding lines run with the tungsten, (lenthwise) allows the current to follow to the tip, thus keeping better control of your heat/
I hope this makes sense. I can explain it a lot better verbaly rather than typing it. LOL
Mike