In all honesty, I don't usually flash in safe mode either, but I know it is recommended, as a precautionary measure. His posted burn wasn't to great for the media, so I thought possibly a bad flash.
I wasn't sure if chachi was asking for help with problems with the 1640 or problems playing back his burns on a stand alone, but I think it's the stand alone that won't playback?
Will your burns play back on the 1640, or is it just your stand alone that is giving you these intermitent problems? If they play back ok on your computer, try giving your player a cleaning.
I have had problems with my stand alone, and my kids have had the same problem with their PS2 occasionally not recognizing disks or freeze ups due to dust occumulating inside the player.
I use a disk type cleaner from WalMart for the stand alone, but it says not to use it on game consoles.
I blow the PS2 out with compressed air from the exit vent in the back, with the tray open. (so the dust can escape)
I am amazed at the cloud of dust that comes out, only after a couple of months.