Glad the volume control is moving, that means it is not a hardware issue.
I am trying to think of anything that might try to control the volume without your input. About all I can come up with is a communications device, but that will normally mute it completely.
If the problems are intermittent, it might be best to try to find something common about your system when this event occurs. If you start your system without running any other software, does it have the problem?
Do you run some game that might effect the volume?