zed 24 volume surge

hello, i have a zed 24 which has a volume surge issue. voulume in the speakers surges from normal to very loud. i am trying to figure out cause. will be disasembling to clean all mecahnical switches etc, but should i be looking at anything else

Do the meters show the level surge?
What outputs are you using?
Can you hear the level surge in headphones plugged into the mixer?