Thank you for your investigation Paeryn, it seems you're right about division/reciprocal being the problem.
I wonder if this is a hardware issue or if it's the sort of thing that can be fixed in drivers?

Hi guys, I'm trying to write some shaders here, but I'm running into some funny behaviour with the glsl mod() function. I was hoping someone with more knowledge of Videocore than me could tell me what's going on here. The code below is run over the whole screen a la shadertoy.com, and explains itsel...