Problem with speakers

I have a 5.1 speaker setup for my PC, but the sound keeps fading/ getting louder intermittently. It must be the speakers as I never have this problem with headphones (tried several pairs and no probs). I don't know much about speakers, so does anyone know what it might be and if its fixable? If not can anyone recommend some good 5.1 speakers for music and games (music most important, need good sound. Would be willing to go down to 2.1 speakers for sake of music quality at the same price) for around £100. Many thanks!