Are they wilting only on sunny days? If so, and they don't wilt on cloudy days, I would strongly suspect their roots are being attacked by some soil-dwelling creatures. My cauliflowers did this due to root maggots. I'm now having that same issue with zucchinis and I suspect Symphylans. If I reach down and grasp the base of the plant and pull on it, I can tell that it would come out of the ground without too much effort. Meaning the roots are very compromised. With my cauliflower plants, I could just lift them right out of the ground and examine the brown, knarly root remains. I don't know what would attack bell peppers in your neck of the woods. I would try tugging on the base of the stem to see how firmly anchored they are in the soil. If you can spare one plant you might dig out the whole rootball and examine it thoroughly. You lose a plant but you may gain some knowledge.