Have you checked :
Did the Disavow Tool work for you? [webmasterworld.com...]
Matt Cutts on stages of Penguin recovery [webmasterworld.com...]
There's a few claims of success in the mix, amongst some who have not. Overuse od exact match and commercial terms in the anchor text is another risk - but usually those point towards the same remedy. Remove or disavow them. Changing them assumes they are not given editorially freely.