I can’t set my avast settings to custom. It always gets back to High settings. 
What are you doing to try to set it to Custom ?
Simply setting it to Custom doesn’t change the underlying scanning function so it would drop back to High.
Why do you want to set it to Custom ?
Making some changes inside the Customize button may well set it to default anyway.
I would also ask why you set it to High in the first place as Normal is the default setting, which provides a good balance between performance and protection.
When you left click the blue avast! icon a window will open that says Resident Protection with a slider below it. If the window looks different, click the button at the bottom that says Less Detail and the window will change. Make sure the slider is set to Normal. This will change all your modules to Normal. Then click the Details button and look at the individual modules, they should all be normal. You should then be able to change them independently if you want to. If you set any of them to High you may notice that the main slider moves to the middle, this is normal. After you do this, if the individual modules keep changing to High on their own there’s a larger problem.
I think the only way it will change to Custom is if you change something in the Customize options, then it should change to Custom automatically.
I noticed the same thing happening to me – that it would change from normal to another setting on it’s own. I’ve changed it back to normal, but it changes on it’s own again, so I just let it do it’s thing. Perhaps it has to do with other programs (like firewall) or behaviors it encounters on your machine.