Flickering artefacts - coloured squares

[SeanNorris]

Hi !
First, sorry for my bad English.
Recently, I reinstall my computer, and now my very simple shaders give me strange flickering artefacts in form of coloured squares.



Before re-installation, I didn't have those artefacts. If I remove the code portion that bind to shader and set its Uniforms, I have no problems.
Here are my shaders:

Vertex shader:

#version 120
varying vec3 lightVec;
varying vec3 normal;
 
uniform vec3 lightPosition;
uniform float lightIntensity;
uniform int lightOn;
 
void main(void)
{
    gl_Position = ftransform();
    normal = normalize(gl_NormalMatrix * gl_Normal);
    lightVec = normalize(gl_NormalMatrix * lightPosition);
    gl_FrontColor = gl_Color;
    gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
}

Fragment shader:

#version 120
varying vec3 lightVec;
varying vec3 normal;
 
uniform float lightIntensity;
uniform int lightOn;
uniform sampler2D tex;
 
void main(void)
{
    float intensity;
    vec4 texel;
    vec3 baseColor;
    vec3 fragColor;
 
    baseColor = gl_Color.rgb;
 
    texel = texture2D(tex,gl_TexCoord[0].st);
    baseColor *= texel.rgb;
 
    intensity = max(dot(lightVec,normalize(normal)),0.0) * lightIntensity;
 
    fragColor += baseColor;
    fragColor += (gl_FrontMaterial.diffuse).rgb;
    fragColor += (gl_FrontMaterial.ambient).rgb;
 
    if(lightOn >= 1)
    {
	 fragColor *= intensity;
    }
 
    gl_FragColor = vec4(fragColor,1.0);
}

Is it a common kind of artefacts when beginning GLSL ? Is it really a mistake from my part in shaders ? Or could it be a problem in uniforms settings in my program ?
Thanks in advance for your help.