Oh sorry, I missed that bit about bump in your original post... I think you put it in after I posted. Sneaky! :O)
I'm a bit lost with this one then. Is it the same if you move the camera in close? Or if you switch texture filtering on/off? Those are other things I would also try.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)