Mobile Android Browser Hijacker

Hi,
I have a Elephone P8000 which is running Android 5.1 and Avast Mobile Security (Version 5.0.18-9064-955eff3). Until recently it was working very well but now I have a major problem with a browser hijacker. The hijacker keeps opening a new page/tab on my browser that then goes to various unasked for pages including gaming sites and Google Playstore apps. It also downloads virus infected .asp files (which Avast identifies). This happens with the built in browser and eventually any other browser that I try.
As I have a fixed data allowance this hijacker is using up this allowance at my expense.
I have tried various solutions that are on the internet, but none works. This seems to be a rapidly growing problem judging by the number of forums discussing the problem.
Avast mobile security does not appear to pick up the problem.
Ideally an add-on or a specific Avast tool that will solve the problem is required.

Can anyone help?

Thanks

Delete the browser.
Check for applications that are not standard installed by Android and that you haven’t installed.
If you find any, delete them ofcourse.
Delete all temporary (cache) files.

See if this solves the problem.