I have a shellstorm auto darking helmet that i've used for about the last 7 yrs, really only maintenance i ever do is change the batt and lens in it. You really shouldnt have to clean the sensor since its under the outer plastic lens. Check your sensitivity setting on it (if yours has one) and set it at the most sensitive setting for spot welding.

Hopefully this isnt the problem but i did have one that went bad after about 4 yrs and when the sensor goes bad it will shit itself off without warning.

BG