@dorkster 
Just to clarify: Running on 9394f16ca6b905 (Fix Linux build), it doesn't matter for whether there is 
prio set to 0 or 1. It's working like a charm, no flickering.

However, the crucial point is it is flickering without assigning a well set prio within the NPCs.
So commenting out the line 

    r.prio = 1; 

in NPC.cpp Renderable& NPC::getRender()

is which worries me.
If you add a printf to output the prio instead, it will yield some very random numbers. And this is worrying me. Where do these come from?