You have either used too much thermal adhesive, have an air bubble under the LED, or do not have good contact between the LED and the heatsink.
Remove the LED, and replace it with another LED. Clean the heatsink very well, and install the new LED with a minimum of thermal adhesive. Rotate the LED in the bed of adhesive to remove any potential air bubbles, and get the adhesive as thin as possible while avoiding continuity with the heatsink. Use your DMM to check before the AA is cured completely. Check again before applying power.
I have used many Seoul LED's at 1A and 1.2A without any thermal issues. The light running at 1.2A is my Mag with a Flupic, and I have used it for 45 minutes+ on high. The 1A lights are mostly Pierce M10's, and again, no thermal issues/tint shift. With proper heatsinking, the Seoul P4 is a killer LED.