I kinda think it's just the nature of these old Mustangs. Mine wanders around some, but that might be because of my ball stud being worn out. The chassis on these cars is inherently weak by today's standards, so your car could be slightly warped or is constantly twisting, leading to unpredictable handling. If subframe connectors don't take care of it, I'm at a loss. Or a leaf spring could be warped on one end or something like that.