Mike,
Opening your controller is a great way to void your warranty. Also, last I recall, you just click the left thumbstick to sprint, you don't hold it down to sprint. Thirdly, it really does help to flash your controller with the button test code and post your results.
Sometimes customers pop open their controller thinking they were going to swap shells or do some other custom work (voiding the warranty), are pissed when they accidentally ripped wires off opening the mod, then make up stories to get a free repair.
In any case, if there really is a problem with the solder joint to the left thumbstick we'll gladly make the repairs, I replied to your support ticket with the details. We'll get the soldering iron hot and ready.
E