Using shader modifiers to animate texture in SceneKit leads to jittery textures over time

I'm writing a scene in SceneKit for iOS.

I'm trying to apply a texture to an object using a sprite sheet. I iterate through the images in that sheet with this code:

happyMaterial = [SCNMaterial new];
happyMaterial.diffuse.contents = happyImage;
happyMaterial.diffuse.wrapS = SCNWrapModeRepeat;
happyMaterial.diffuse.wrapT = SCNWrapModeRepeat;
happyMaterial.shaderModifiers = @{ SCNShaderModifierEntryPointGeometry : @"_geometry.texcoords[0] = vec2((_geometry.texcoords[0].x+floor(u_time*30.0))/10.0, (_geometry.texcoords[0].y+floor(u_time*30.0/10.0))/7.0);" };

All is good. Except over time, the texture starts to get random jitteriness in it, especially along the x-axis.

Someone mentioned it could be because of "floating-point precision issues," but I'm not sure how to diagnose or fix this.

Also: I'm not sure how to log data from the shader code. Would be awesome to be able to look into variables like "u_time" and see exactly what's going on.