Wharf Rat , I had the exact same problem with my 1974 Raleigh frame. I discovered that the stays on one side were slightly longer than the other. I doubt this is the case with your Miyata because they are built better than the 1970's era Raleighs. Take a look at this thread to see how I handled the problem...

1974 Raleigh Grand Prix Rehab...Part 5...Frame Alignment