Wille,
There are a number of reasons a shotgun like your's might get loose. A worn hinge pin like ocharry already pointed out, a worn locking bolt, worn, loose or cracked barrel lumps etc.
Contact Ithaca Gun Repair aka Diamond Gunsmithing I'm sure they can help.
211 Dey Street
Ithaca, NY 14850
607.273.4510
http://ithacagunrepair.com/