I'm really sorry about this issue guys. We're trying to resolve it as quickly as possible but sadly it's very difficult to track these down. They're most prevalent in the evenings when there's less competition for ad impressions so they're more likely to win.
Those seeing this issue over the past couple of days, we'd really appreciate it if you could please share which device and browser you're using? Also, if you're on Wi-Fi or mobile data, that would be useful too. Please feel free to pm me those details if you don't want to share them publicly.
We're working on a number of ways to stop these entirely, but it's always a bit of a cat & mouse game sadly.
Apologies to those affected and thank you for your help!
From the Website wish list and problems thread