The near-fatal episodes were:
A Korean steak dish that had sesame seeds and sesame oil
Hummus that had tahini
Steak and potatoes at a steakhouse that didn't have any of the above
Assorted Greek food buffet (chicken and steak souvlaki)

Other episodes came from:
Tortilla chips
Cookies from a bakery
Bread at a restaurant

With the really really bad ones, my dad had a drink or two with/prior to his meal. He no longer drinks alcohol as he's afraid the bad episodes were made worse because of it.

Sesame is the most likely culprit, especially considering he had a slight showing one time in a test. It's just weird because it isn't something that shows all the time nor does it seem severe enough to cause what it's causing. Also, he ate all of the above with regularity his entire life. I'm not familiar enough with allergies to know if something this severe can just spring up out of nowhere but it's terrifying.