Im wondering if the varying factor here is the installation procedure. If it were me, I would bolt up everything, but keep everything totally loose. Bolt up the crossover, snug it up loosely, then start to snug up the header, then go back to the crossover and tighten a little more, then back to the headers and tighten them a little more, so on and so on. Ive had to do this on a few exhaust installations in the past when nothing seemed to want to go on right. Does the problem lie in angular problems or a flange not being where it should be, along the axis of the pipe?