Lightmapping bake glitch - white flashing blocks

Hi, Has anyone had this problem with Light Baking in Unity?

Once a bake has been complete and you run the game, these little white blocks appear, flashing, randomly. I’ve done stuff like check seams, UV maps etc but I can’t find a definitive answer or solution.

If anyone can shed light on this it would be much appreciated!
Using Unity 5.2.1f1

Just to post an update, here’s the problem shown in screen capture:

Has anyone else had this problem? Thanks

@electrolyte @NGC6543 The Directional Specular option is twice as expensive than the base Directional. If you find you need better performance, another solve is to uncheck the ‘Compression’ box. Around here we call those ‘Squares of Death’, and it appears they are a compression artifact of sorts.