nausea and vomiting 5 hrs after eating

I have a condition that occurs without regularity, but very troublesome when it shows up.  4-5 hrs after eating, I begin to burp and to feel nauseated, and then vomit.   It doesn't seem to be any particular food that sets it off.  The symptoms are worsening now, with several bouts of vomiting, and the nausea lasting a few days.  Sometimes it will not happen for months, or even a year.  But then recurs.  I have been checked for gall bladder, pancreas, and liver and all were ok.  Do you have any answers?