Your problem is that youíre using a butt weld. They have their uses such as doglegs and pillar posts, but are not the best choice all around. Because metal flexes it tends to bend at the weakest joints. I use an overlap weld without flanging. It provides a stronger weld hold without flex fatigue associated as evidenced with a butt weld.
Iíve provided two photos of an Acura I sectioned a quarter panel on last week.