Periodic excessive salivating and GI pain

I am a healthy 32 year old male.  For the past five years, every 4 or 5 months I will have a sudden, inexplicable pain being to rise in my stomach/upper GI tract.  It isn't sharp, and it isn't like nausea, though it feels like I need to vomit.  At the same time, my mouth will start producing copious amounts of saliva.  It is almost impossible for me to speak during this time.  The spell lasts for around 3 minutes and then disappears, rarely to return for another few months.

I travel extensively and the first spell happened in Sydney, Australia in 2004.  Today I had another attack, this one more severe than any previously.  I was out walking in a park and had to suddenly lay down on the ground and put pressure above my stomach, massaging downwards slowly, while saliva poured out the side of my mouth.  When the pain would leave, I would stand up and it would suddenly return, forcing me to lay down again.  After 4 attempts to stand up, I laid there for about 10 minutes, massaging my stomach and it finally passed.  3 hours later, another minor spell returned briefly, and was gone in 30 seconds.

I have never been able to associate this with any type of behavior or dietary issue.  It happens randomly, always during the day.

Due to extensive travel, I have been treated before for intestinal parasites/giardiasis.  This treatment has never had an impact on this condition.