Well, now that you say wilt, a bell went off in my mind. I'm wondering.... versillium wilt? Bacterial wilt? The way to diagnose this is to slice open the stem; if the inside of the stem is blocked, dried up and funny looking it's wilt. Try doing this, cut the stem in half, look at it, then try to post a pic of it. I've had tomato plants die of wilt: if it is wilt, I will recognize it in the pic.