Is it only certain keywords, or any time you visit your site from Google? They could've slipped a little redirect in one of your WP theme files that sends anyone referred from Google to the other site. Not hard to do with php. You should have a look at your theme's template files for any suspicious code which may have been inserted.the site is not new, and ranks for many keywords in google, however for a few particular keywords you are redirected to an alternate site.
How do i stop this?
Reuploaded my theme, worked like a charm. Thanks guys