I'm starting to get weird browser behaviour. Mainly on specific websites, including some pages on SB

Eg when I try to login to Bankwest and/or NAB online banking, the first time the pop-up login window opens it quickly diverts to an ad (Sportsbet this time with a URL
http://rbv.jobfindgold.info/a1/..........). But if I close that popup window and do exactly the same again (click login) I get a normal login window.
Some on SB. Some specific forum topics seem to cause a popup window that says "The page at shuang 11temai.com says Congratulations!!! You are todays locky winner of some Apple i-product".
I've always had free AVG and recently upgraded to a 30-day-trial of the paid version. I've run a scan and it doesn't find anything.
Any tech folk out there know wtf is happening with my computer please?