lines that shouldn't exist!

Hi, I wrote a program that simulates radiography, I use 3d texture as volumes,GL_QUADS to map the slices of texture and glfunc(add) to blend the quads. the program works but i see some strange lines, anyone knows which could be the problem? I tryed to draw the quads more closest but I always have the problem… here is an example(i’ve highlighted examples of the wrong lines):

Please don’t crosspost in the beginner and advanced forum.

Have you try on different computer?
I’m really not sure but this could common from a graphics cards malfunction, like a texture that don’t work well anymore. I had the case once… that was a bit strange.

I’ve tried the code on another pc but it’s the same thing…

maybe some kind of texture interpolation problem?