[PyOpenGL-Users] shader errors

[Michael L. <mic...@gm...>]

Hi! i have some very strange behaviour with my shader.
the code i took here:
http://www.geeks3d.com/20091013/shader-library-phong-shader-with-multiple-
lights-glsl/

[CODE]
program = compileProgram(
        ['''
varying vec3 normal, eyeVec;
#define MAX_LIGHTS 8
varying vec3 lightDir[MAX_LIGHTS];
varying vec2 texture_coordinate;
uniform int numLights;
void main()
{
  gl_Position = ftransform();
  normal = gl_NormalMatrix * gl_Normal;
  vec4 vVertex = gl_ModelViewMatrix * gl_Vertex;
  eyeVec = -vVertex.xyz;
  int i;
  texture_coordinate = vec2(gl_MultiTexCoord0);
  for (i=0; i<numLights; ++i)
    lightDir[i] = vec3(gl_LightSource[i].position.xyz - vVertex.xyz);
}
        ''',],
        ['''
varying vec3 normal, eyeVec;
#define MAX_LIGHTS 8
varying vec3 lightDir[MAX_LIGHTS];
varying vec2 texture_coordinate;
uniform sampler2D my_color_texture;
uniform int numLights;
void main (void)
{
  vec4 final_color = gl_FrontLightModelProduct.sceneColor;
  vec4 texel = texture2D(my_color_texture, texture_coordinate);
  vec3 N = normalize(normal);
  int i;
  for (i=0; i<numLights; ++i)
  {
    vec3 L = normalize(lightDir[i]);
    float lambertTerm = dot(N,L);
    if (lambertTerm > 0.0)
    {
      final_color +=
        gl_LightSource[i].diffuse *
        gl_FrontMaterial.diffuse *
        lambertTerm;
      vec3 E = normalize(eyeVec);
      vec3 R = reflect(-L, N);
      float specular = pow(max(dot(R, E), 0.0),
                           gl_FrontMaterial.shininess);
      final_color +=
        gl_LightSource[i].specular *
        gl_FrontMaterial.specular *
        specular;
    }
  }
  gl_FragColor = final_color*texel;
}
        ''',])

#after this ->
if program:
                glUseProgramObjectARB(program)
                num_lights = glGetUniformLocationARB(program, "numLights")
                glUniform1iARB(num_lights, 4)
[/CODE]
######################################################

it's ok at work but at home it crashes (win x64, geforce 7600 GT). Other shaders 
work fine - i found that it crashes somewhere near for-loop
for (i=0; i<numLights; ++i)
    lightDir[i] = vec3(gl_LightSource[i].position.xyz - vVertex.xyz);
The main question is can i somehow debug shader and print some log what's going 
on?
and how it can be that everything is ok at one machine and invalidOperation at 
other?..
thank you!