LED Rear Indicator Lights

[Don262]

Anyone tried to replace bulbs with Leds. I have but whilst they work they are frantic,flashing at roughly twice the rate just as if a bulb has failed, and the indicator arrows on the dash both come on at the same time. I have tried to put a resistor (of various ohms) in parallel but it makes no difference.
Any budding auto electricians out there with a suggestion of what might work?
The Leds I have used are:
955 13 LED Extreme Power 2W 360 degree - White. Anything bigger would probably not fit in the light fitting.
Cheers
Don