Have Involuntary Regurgitation. Endoscopy Shows Stomach Mucus. Cause?

I have a problem of what I call involuntary regurgitation. When I eat my food comes back up with no warning. It happens as long as 6-7 hours after I eat. For example if I have salad for lunch at 11:30, when I get home from work and go to do yard work and bend over the salad lettuce I had for lunch comes up. I have lots and lots of mucus with it. This has gone on for over 10 years. I have had colonoscopy and 3 endoscopies and nothing showed up except for the dr to tell me that I have an awfully lot of mucus in my stomach. I don't have heart burn, I don't have acid reflux. I just have food that constantly and without warning comes back up into my mouth. It is mixed with lots of mucus. After being seen by family dr and gastroenologist several times we came to the conclusion that if it was going to kill me I would have been dead a long time ago so just live with it. However, I would like to know what causes it and why I have so much mucus in my stomach. I'm 66 years old. In very good health. Take no medicine. Am more active than most 30 year olds, work full time, and never am sick. Just tired of dealing with this problem. If I'm not where I can spit the mouthful of garbage out I have to swallow it back down. Sounds gross but have done it for so long it does't bother me anymore. Thanks for listening,,,Christine