Photoshop :: Odd Photo Merge?

For the past couple of years Photomerge in PS3 and PS4 has been sufficient for 95 percent of my stitches which are generally simply 2x flat stitches from shifting a T/S lens. These are all building interiors and exteriors with plenty of common points and overlap. This is an important part of my workflow and I do a few of these a week for clients. Up till recently they merged flawlessly. All of a sudden Photomerge can't seem to align any images giving me the error message "some images could not be automatically aligned". This happens under all the different merge modes. This also happens from Bridge or from open files in PS. It literally cannot merge any set of images. This is with the camera always on a tripod and exposure and WB locked down.  The only thing that has changed is I have been playing with memory usage and cache levels trying to improve performance, but even after returning them to the original settings Photomerge will not complete a merge. It matters not what merge mode you use or whether you are working from raw files or tiffs or from Bridge or open files. I am having to do every stitch in Autopano Pro which slows down my workflow considerably and frankly does not work as well on keeping the lines straight.  Widows XP with a huge scatch disk dedicated just to PS. A reinstall?