deferred rendering and gaussian blur - artifacts

Question

I compute Gaussian blur in two passes (horizontally and vertically). Shaders look like this:

Horizontal blur - fragment shader:

#version 420
layout (location = 0) out vec4 outColor;
in vec2 texCoord;
float PixOffset[5] = float[](0.0,1.0,2.0,3.0,4.0);
float Weight[5] = float[]( 0.2270270270, 0.1945945946, 0.1216216216, 0.0540540541, 0.0162162162 );
float scale = 4.0; 

uniform sampler2D texture0;
uniform vec2 screenSize;

void main(void)
{
    float dx = 1.0 / screenSize.x;
    vec4 sum = texture(texture0, texCoord) * Weight[0];
    for( int i = 1; i < 5; i++ )
    {
        sum += texture(texture0, texCoord  + vec2(PixOffset[i], 0.0) * scale * dx ) * Weight[i];
        sum += texture(texture0, texCoord - vec2(PixOffset[i], 0.0) * scale * dx ) * Weight[i];
    }
    outColor = sum;
}

I use deferred rendering and the following screens shows a diffuse material texture after blurring. I simplified a render loop for the sake of clarity(only one render target - a diffuse material).

A render loop - first case:

• bind fbo1
• gbuffer stage

• unbind fbo1

• bind fbo2

• read a diffuse texture, render to a temporary texture (full screen quad)
• read a temporary texture, horizontal blur, render to a temporary texture (full screen quad)
• read a temporary texture, vertical blur, render to a temporary texture (full screen quad)

• unbind fbo2

• read a temporary texture, render to the default framebuffer (full screen quad)

The final image has artifacts(flickering pixels). Some of them are placed between two triangles which create the full screen quad.Screen1

A render loop - second case:

• bind fbo1
• gbuffer stage

• unbind fbo1

• bind fbo2

• read a diffuse texture, render to a temporary texture (full screen quad)
• read a temporary texture, horizontal blur, render to a temporary texture (full screen quad)

• unbind fbo2

• read a temporary texture, vertical blur, render to the default framebuffer (full screen quad)

Some artifacts may appear between triangles: Screen2

A render loop - third case:

• bind fbo1
• gbuffer stage

• unbind fbo1

• bind fbo2

• read a diffuse texture, horizontal blur, render to a temporary texture (full screen quad)

• unbind fbo2

• read a temporary texture, vertical blur, render to the default framebuffer (full screen quad)

The final image doesn't contain artifacts.Screen3

How to fix that ? The above code is simplified but normally the first case(render loop order) is most useful, for example: I want to blur a glow texture and use it when shading.