What Causes Chest And Shoulder Discomfort?

I am 65 years old,male, had a heart attack in 1990, last year, I'm experiencing some weird things in my chest and shoulders while walking , but it will be gone if I will rest a while and after belching. Thru ecg ,the doctors said that there's no more sign on the previous heart attack, but what they show me on 2D echo is my heart is only functioning 30 %. I do my regular walk, but there times that I feel some discomfort in my chest and shoulders and will be gone after belching , and this happens also even resting but under stress. I felt comfort when gas get out from my body. What could be the cause and what could be done? By the way the doctor said a year ago, there's no treatment except angioplasty or by pass after undergoing angiogram.