choking on food

My dad is in his late 50's.  Sometimes when he eats certain foods like rice or chicken for example, get stuck in his throat.  He can still breath, but large amounts of saliva build up and if he doesn't keep spitting it out he can't breath.  This has been happening since I was young.  We'll race him to the emergency room and the food will just end up going down and he'll be fine, or he'll throw it all up and he'll be fine.  They have done a barrium swallow on him and have found nothing wrong.  This is so scary every time it happens and my question is: is there any other tests that we can do or any suggestions about why this is happening.