What apps did you install or update shortly before this started?
Any apps installed as apk downloads from random websites, as opposed to from the Play Store, are more likely to contain adware (though there is adware in the Play Store too).
If you can identify which of your apps have the privileges to e.g. draw over other apps they may also be more suspect (something like Addons Detector may help).
However you track it down, the trick is to find the app that's causing these and remove it.
Please tell us if you made any other changes, such as using a one click root tool, since such things could also be relevant.