Trust Relationship with workstation to Server.

Windows 7 Workstations loses Trust Relationship with Server 2012. Had this happen on 15 percent of the workstations when a new server was installed. Some worked fine for a few weeks. Suddenly the message.(User can't log into the domain - gets message) I take from domain, change to workgroup. change back to domain. Restablishes the trust relationship, but, darn creates new user (same name but User.doman.001 on the workstation. Bummer. Why, and best solution to fix?

Thanks - Michael