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[GLSL] spotlight problems

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My per-pixel spotlight shader isn't working right. The lit area moves around when I move the camera even though the light's position doesn't change. Even then, the lit area distorts strangely as it moves over the geometry. Here's the vertex shader:

#version 130

uniform mat4x4 projectionMatrix;
uniform mat4x4 modelViewMatrix;
uniform mat2x2 textureMatrix;
uniform vec4 globalAmbient;
uniform vec3 lightPosition; // in view space
uniform vec4 lightAmbient;
uniform vec4 lightDiffuse;
uniform vec4 materialAmbient;
uniform vec4 materialDiffuse;

in vec4 position;
in vec2 textureCoordinate;
in vec3 normal;

out vec2 vertexTextureCoordinate;
out vec3 vertexNormal;
out vec4 vertexGlobalAmbient;
out vec3 vertexLightDirection;
out vec3 vertexHalfVector;
out float vertexLightDistance;
out vec4 vertexAmbientColor;
out vec4 vertexDiffuseColor;

void main()
vertexNormal = mat3x3(modelViewMatrix) * normal; // We assume no non-uniform scaling here

// Compute the light direction
gl_Position = modelViewMatrix * position;
vec3 lightVector = lightPosition - gl_Position.xyz;
vertexLightDirection = normalize( lightVector );
vertexLightDistance = length( lightVector );
vertexHalfVector = normalize( vertexLightDirection + gl_Position.xyz );

// Compute ambient and diffuse colors
vertexGlobalAmbient = globalAmbient * materialAmbient;
vertexAmbientColor = materialAmbient * lightAmbient;
vertexDiffuseColor = materialDiffuse * lightDiffuse;

gl_Position = projectionMatrix * gl_Position;
vertexTextureCoordinate = textureMatrix * textureCoordinate;

and the fragment shader:

#version 130

uniform sampler2D texture;
uniform vec4 lightSpecular;
uniform float lightConstantAttenuation;
uniform float lightLinearAttenuation;
uniform float lightQuadraticAttenuation;
uniform vec3 lightDirection;
uniform float lightConeAngleCosine;
uniform uint lightRateOfDecay;
uniform vec4 materialSpecular;
uniform int materialShininess;

in vec2 vertexTextureCoordinate;
in vec3 vertexNormal;
in vec4 vertexGlobalAmbient;
in vec3 vertexLightDirection;
in vec3 vertexHalfVector;
in float vertexLightDistance;
in vec4 vertexAmbientColor;
in vec4 vertexDiffuseColor;

out vec4 fragmentColor;

void main()
fragmentColor = vertexGlobalAmbient;
vec3 normal = normalize( vertexNormal );

float normalDotLightDirection = max( dot( normal, normalize(vertexLightDirection) ), 0 );
if( normalDotLightDirection > 0 )
float spotEffect = dot( normalize(lightDirection), normalize(-vertexLightDirection) );
if( spotEffect > lightConeAngleCosine )
spotEffect = pow( spotEffect, lightRateOfDecay );

float attenuation = spotEffect / ( lightConstantAttenuation +
lightLinearAttenuation * vertexLightDistance +
lightQuadraticAttenuation * vertexLightDistance * vertexLightDistance );

fragmentColor += attenuation * ( vertexDiffuseColor * normalDotLightDirection + vertexAmbientColor );

vec3 halfVector = normalize( vertexHalfVector );
float normalDotHalfVector = max( dot(normal, halfVector), 0 );
fragmentColor += attenuation * materialSpecular * lightSpecular * pow( normalDotHalfVector, materialShininess );

vec4 texel = texture2D( texture, vertexTextureCoordinate );
fragmentColor *= texel;
fragmentColor.a = texel.a;

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