I've built three Zetex 300 based boost converters to power Luxeon LED's, using hand-wound inductors. Two of them work perfectly, driving the LED at 2.9V/350mA as designed. The third works most of the time, but occasionally the LED starts flickering - it momentarily dims once every 3 seconds or so. It does this for a couple of minutes, and then settles down to a steady light. It sometimes does this strange flickering several times in an hour, and other times it can run for hours without any problems. Any ideas what might cause this behaviour? Ambient temperature and battery state don't seem to have any effect.