Some signs (banners, chests, skulls) are randomly flashing. It's like light changes only for that signs (...) .
it depends on my postion and direction (maybe only other chunks).
I hope the video describes the bug well.
I cant reproduce that for other worlds, but it happens again after a restart for this world.
https://www.youtube.com/watch?v=5SYxlojQKEc
Steps to reproduce:
Place some signs, banners, chests, skulls
Place an item spawner
/setblock ~ ~ ~ minecraft:mob_spawner 0 replace {SpawnData:{id:Item,Item:{id:"minecraft:iron_ore"}}}
Code analysis by @unknown is found here
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updated to driver version 15.200.1062.0
from the 15.07.2015
So is it still an issue?
yes, it also appears for chests, banners and skulls

Confirmed for 1.9-pre4, there must be an item spawner near by
/setblock ~ ~ ~ minecraft:mob_spawner 0 replace {SpawnData:{id:Item,Item:{id:"minecraft:iron_ore"}}}

Please link to this comment in the description
The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.
The reason why this happens is that the public void doRender(EntityItem entity, double x, double y, double z, float entityYaw, float partialTicks)
method of the net.minecraft.client.renderer.entity.RenderEntityItem
class is now calling the public static void enableStandardItemLighting()
method of the net.minecraft.client.renderer.RenderHelper
class (this used to not be the case in 1.8) which is supposed to "fix" MC-92915, however as MC-93642 is not fixed by this change, the underlying issue might be a different one.
The reason why this is only happening for spawners and not for items dropped on the ground is very likely that the method is called while tile entities are rendered.

Still in 1.9.1-pre1

Still in 1.9.1-pre3
Outdated AMD graphics card drivers, update from http://www.amd.com, see also MC-62958