I think there are some magic hotkeys that will get you a Linux prompt at boot time... might be possible to catch one of the frozen systems and see what's going on and/or investigate the log files. Linux can be pretty verbose about what it's doing at boot up and that might show something. Btw, have you tried leaving one of the 'stuck' systems on the splash page for ~30 minutes to be sure it's not doing something dumb like a filesystem check that takes a while? I had that happen once on a headless system.