Some more info about what's going on would be helpful. Like pjandyho asked, what battery are you using? What light? Is the battery fresh/fully charged? How long have you had the light, and how much use has it gotten? What current gets sent through it on the mode(s) you use?
(FYI, LED stands for Light Emitting Diode, so to call it an LED diode is redundant ;) )