How are you so certain it is cross linking which caused the trouble? There have been many changes in the SERPS in december and januar and when one gains, another goes.
When your links are relevant, and it seems to me like that, there shouldn't be much of a problem. I do it too between three of my sites. I don't think it will give me better results in the SERPS but I never noticed a negative effect as well. As long it is relevant to the user.
Maybe it is an open door but did you check your pages for near dup. content? Is it possible that is the reason?