You guys are right on the money!
The flasher works by (and is dependent on) the amount of current flow in the flasher circuit. The flasher uses a bi-metal tab that works like a relay contact. As the current flows though the bulbs, heat is generated in the metal tab, causing it to bend and opening the circuit. The metal tab cools and the circuit is re-enabled. Blink-blink-blink.
LED's are much, much more efficient at producing light, therefore are low current devices. They just don't pull the current required to heat up that element properly.
Using a resistor for each taillight blinker should cure your 'quicky-blinky' problems. Just place it in parallel with the LED terminals.
Hope this helps.
Mark