At the risk of being [wrongly] corrected for spelling again as I was in another post about slow leaks, I've had the same issue. I've just gotten use to always making sure the valve is firmly screwed into the stem, and the cap is only lightly screwed down. While it theoretically doesn't do anything, I keep the cap on just to make sure that one in a million stick doesn't hit the valve and unscrew it, or if the valve is loose in the stem, the cap holds it on. Same for the stem washer, it gives the stem a little more rigidity when pumping, or when getting hit by the proverbial random stick, so it's less likely to tear off. Neither is necessary, but then I've never really needed my helmet in 40 years of riding either...