Vomiting 3 1/2 hours after seafood- like clockwork!

Ok, maybe this is my problem too. Not sure. For about 6 years now, I've had a periodic reaction to seafood. Not just shell fish or a creation type of fish, and not all the time either. I don't always drink a glass of wine with dinner but often; at least when ai'm at a restaurant anyway. I've found that the common denominator is restaurant food. This is what happens... Like clockwork, about 3 1/2 hours after eating seafood at a restaurant, I get violently ill and begin to vomit. It may last 30 to 60 minutes, until there is nothing left in my stomach. Think is... It would only happen with about 20-30% of the seafood dinners I ate at restaurants. And never at some of the restaurants. I would never get sick from seafood at the Outback steakhouses or at my country club. I've noe totally quit eating seafood because it's not worth the risk. I'm honking it's a newer chemical used in the handling or seasoning of seafood in some restaurants. Or may e even some leaning product used by one or more seafood suppliers. I'm not always drinking when it happens. And it has only happened once when cooking fish at home. I went to a Gastro-intestinal doctor who scoped and tested my stomach. She did find that I had a couple stomach ulcers but nothing else. Perhaps the ulcers made me more sensitive to "whatever". Who knows, but they're gone now so I tried just 1 little seafood taco last week (at a restaurant of course) , and again, like clockwork, 3 1/2 hours later I was hanging over the toilet spilling my guts. I feel ok a few hours later. Help!!! I always loved seafood!!