Hi...Sometimes when I eat, I get an air or gas bubble in my esophagus that prevents food from going down into my stomach. It is usually directly under my sternum. Sometimes, it even brings food back up. It is not painful, but aggravating, because I have to wait for it to dissipate before I can continue to eat. If I make myself burp, sometimes it will help move the bubble along. What causes this? And what can I do to prevent this?