From what I have been reading, you should rotate your food and not try to eat the same thing everyday. I have the same problem as you, so I have to eat some grains to keep weight on because I don't eat meat.
I have rotated quinoa, brown rice and beans. I haven't tried buckwheat. I allow myself about 1/4 cup of each of one of these per day. My symptoms keep getting better, so I find that this rotation works for my body. I'm not an expert, but that is what has been working for me. So... if you have a couple grains that don't aggravate your symptoms, just rotate them and try not to eat too many carbs.