Signature link drops google SERP ranking

[kevcj]

For about 6 months I had my wifes forum and my sons forum on the same server and with the same IP address. A few weeks ago I moved the sites to a new server and gave them different IP addresses.

On my sons forum, when I add a link to my signature that goes to my wifes forum, my wifes forum drops a full page in the google results page. If I leave the link in my signature, the site keeps dropping further and further back.

When I take the link out of the signature, a few days later (maybe a week or so), the site regains its ranking.

Both sites are between 6 - 8 months old.

On my sons forum I have over 700 post, and would like to use those signature links to help target certain keywords on my wifes site.

I am going to guess that google might might be giving the links a penalty because the sites were on the same IP address? Or maybe a penalty because of so many new links over night?

Any suggestions or ideas as to what the problem might be?