Have you looked into chondritis? (sp).
Hi!
Pain occurring intermittently in left chest or left arm or in back should be investigated for pericarditis (inflammation of lining of heart), heart pain and angina. A 12 lead EKG or ECG should be done. Other than this the pain can be due to pinched cervical spinal nerve, acidity, gastric ulcer, or due to shingles.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your PCP for primary examination followed by proper referral.
Take care!