Pufak on "Unwanted Ads opening in new window"

I administer website http://www.salmonplus.com
Page opens ok on entering URL, but on clicking any page tab, an unwanted advertisement opens in new window.
Only happens once per browser session.
Seems to be IP/browser and time/date dependent as does not happen every time a browser opens, but open a different (other brand) browser and it happens every time.
This site is self-hosted - not on WordPress host.
Different ads open up every time so it is being driven by a database somewhere.
I use few plug-ins: Akismet, NextGEN Gallery, and Sweet Captcha.

Any ideas would be most appreciated.

@rogerhallett :
Please deactive plugin one by one check this issue.
One suggestion set better permalink setting with postname.

Try to reset browser settings to defaults.
OR
resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

Thanks AjitKhodke. But it is not only my browsers - just about everyone visiting the site has the problem. Will have a look at the Plugin folder. Thanks.

Ravi patel. I have disabled and uninstalled all plugins except NextGEN ans Akismet. Will drop Akismet next, but I have several Galleries on the website and very reluctant to drop NextGEN. But I will if necessary.
I don't know what you mean by your permalink setting suggestion. Can you expand please. Thanks.

Hi rogerhallet

I think the problem with that theme sometimes free themes comes with unwanted ad can you try to change the theme and see if popups still comes out ?!

cheers
Pufak