Cross-Linking
There are different views on this, so you will want to ask someone else also to get a well rounded opinion.

Have you ever heard of cross-linking(domains)?
My view on linking to other domains in navigation is that it is seen by search engines as somewhere between grey and black hat tactics. You do seem to be using it in a good way, but I wouldn't use it at all.

Was it this way for the 7 or so year span that you were doing so well?