Probably needs new pots (potentiometers). If you're not confident changing them yourself, ask at the local music shop - they will almost certainly know someone who you can pay to do this for you.
Alternatively, you could attempt to do it yourself - or just clean them. There are many guides on how to do this, e.g, this one: http://www.instructables.com/id/How-To-Clean-The-Controls-On-Your-Amplifier/step5/Inspect-dirty-contacts/