Your LED and resistor is getting too hot... I bet.
If you work out which resistor you need (in ohms), you'll probably also notice a "wattage" rating. The bigger, the more amperage it can handle. The more amperage it has to drop for the LED, the more it has to transfer energy into the form of heat.
The heat will make an LED work, not work, flicker... you name it.
For most 14v applications, a 1 watt resistor is MINIMUM, but i've encountered problems even with those! A 2watt would work, but they're HUGE!
Only workaround I can think of is to use two resistors per LED. One to drop the voltage, and one across the LED to drop the amperage. Theoretically, they will both share the load. 2x1watt = 2watt.
Yet to actually TRY this though. =-[