This is interesting. I have 2 sweet tarts, both in ground for > 5 years. One produces small crops every year, the other has been producing heavy crops, regularly snapping branches. Is it possible that the tree is sensitive to rootstock influence?
On a related note, one of my neighbors has an Edward that produces semi-heavy crops nearly every year. Yet, the branch that I grafted from his tree (onto a keitt) has produced very little. (To be fair it's the first year that it's flowered for me.) I'm a little bummed, cause I actually gave my neighbor the tree. We thought about digging it out of the ground when he sold, err, lost the house to foreclosure (he offered it to us), but it was too big.