When I am online, I get always when I unlock the screen a browser popup with a specified address: adcash.com/... It is just a blank page, but very annoying. I have tried all the Play offers and highly recommended apps, but it is still there.
I also have this issue, and it has just started in the last week.
Do you by any chance have an HTC device?
Edit - I've been uninstalling apps one by one, and the removal that seemed to stop it was Ad Block Plus.