Render to texture - level of detail
===

I've got a problem with my render to texture process. When I render the scene with width = height = 512 it has nearly no errors but the smaller the texture and scene gets, the more errors it gets.
The error is that regions of the texture are black, what makes no sense. 

Here are some screenshots

512*512: http://www.ld-host.de/uploads/images/d9452fa0ba28494830fd96f0f15b9eba.png
128*128: http://www.ld-host.de/uploads/images/a39c141282a622f086d4a96b070a56a3.png

Here is my code how I render to texture and use the texture later

        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
        glMatrixMode(GL_MODELVIEW)
        glLoadIdentity()
        gluLookAt(0,0,200,0,0,-1,0,1,0)
        
        
        self.fbos = glGenFramebuffersEXT(1)
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, self.fbos)
        self.depthbuffers = (glGenRenderbuffersEXT(1)
        glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, self.depthbuffers)
        glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT, width, height)
        glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, self.depthbuffers)       

        self.textures = glGenTextures(1)
        glBindTexture(GL_TEXTURE_2D, self.textures)
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER, GL_LINEAR)
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER, GL_LINEAR)
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8,  width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, None);
        glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,  GL_TEXTURE_2D, self.textures, 0);
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, self.fbos);
        
        glEnable(GL_CLIP_PLANE0)
        glEnable(GL_CLIP_PLANE1)
        glClipPlane(GL_CLIP_PLANE0, (0,0,1,-1 * self.start + self.diff))       
        glClipPlane(GL_CLIP_PLANE1, (0,0,-1,self.start))   
        
        # render the mesh
        glTranslatef(-64,-64,-64)
        glEnableClientState(GL_VERTEX_ARRAY) # Enable something in OpenGL 
        glEnableClientState(GL_COLOR_ARRAY) # Enable something in OpenGL
        glBindBuffer(GL_ARRAY_BUFFER,self.vbo[1]) 
        glColorPointer(3,GL_FLOAT,0,None) # Tell OpenGL that it contains only ColorValues
        #Now the vertex Buffer with positions
        glBindBuffer(GL_ARRAY_BUFFER,self.vbo[0]) 
        glVertexPointer(3,GL_FLOAT,0,None) # Tell OpenGL that it contains the Positions for each Points
        glDrawArrays(GL_TRIANGLES,0,len(self.verts)+len(self.color))    # Merge both

        glDisableClientState(GL_COLOR_ARRAY); 
        glDisableClientState(GL_VERTEX_ARRAY);
        glDisable(GL_CLIP_PLANE0)
        glDisable(GL_CLIP_PLANE1)
        glBindTexture(GL_TEXTURE_2D,0)
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); 

        #Plane for showing texture
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
        glMatrixMode(GL_MODELVIEW)
        glLoadIdentity()
        gluLookAt(0,0,1,0,0,0,0,1,0)
        glClearColor(1,1,1,0)
                  
        glEnable(GL_TEXTURE_2D)
   
        glBindTexture(GL_TEXTURE_2D, self.textures)
        glBegin(GL_QUADS)
        glNormal3f(0.0, 1.0, 0.0)
        glTexCoord2f(0.0, 0.0)
        glVertex3f(-1, 1, -1)
        glTexCoord2f(1.0, 0.0)
        glVertex3f(1, 1, -1)
        glTexCoord2f(1.0, 1.0)
        glVertex3f(1, -1, -1)
        glTexCoord2f(0.0, 1.0)
        glVertex3f(-1, -1, -1)
        glEnd();
        glDisable(GL_TEXTURE_2D)
        
        pygame.display.flip()

How can I achieve a better quality for the smaller texture? The mesh has no holes but the texture sometimes has.