It looks like your study has multiple bodies in it. So, the bracket with the fixture applied is likely not itself unstable but one of the bodies that should be connected to the bracket is unstable. You may want to define specific contact sets to bond these other bodies onto each other and your bracket to stabilize the system.
Otherwise, you could try going through the first of the three diagnostic processes listed here (GoEngineer - Instability Diagnosis ) to try to locate how the study is unstable.
Two reasons for this: lake of fixture or lake of proper bonding contacts
If the above are checked and good, soft spring can help.