Unable to reproduce problem in version 1.8.1, after putting an iron golem in a well-lit room with stone pressure plates on the floor (connected to an alarm if the golem were to disappear) on a super-flat map in creative mode and normal difficulty, and waiting for almost 9 hours.
I've also observed this four times. On one occasion, the iron Golem was attached to a fence via a lead. I was leaving the area and going into the nether, so I'm not sure what triggered the golems to despawn.
Unable to reproduce problem in version 1.8.1, after putting an iron golem in a well-lit room with stone pressure plates on the floor (connected to an alarm if the golem were to disappear) on a super-flat map in creative mode and normal difficulty, and waiting for almost 9 hours.