chronically elevated cpk's

I'm a 48 y/o wm-and for the previous 5 years i have had chronically elevated cpk's ( 100% mm fraction). The range has been 295-1100, usually hovering at 350.Recently I've developed joint pain and stifness, and significant chest pain. Negative cardiac w/u, i.e. stress echo- cardiolyte- ct-angio-.The chest pain can last for days, and begins 1-2 days post light weight exercise.I have not had a muscle biopsy, but am now stongly considering it.What might the differential include here? And what other studies would be feasible? Thanks, yeebin