Thanks to the sugestions from this forum I purchased some of the Tay-yuden
DVD-r disks from Rima.
My batches of Ritek go4's were failing near the end of the disk. Using Nero speed data test I found many bad disks.

These new Tay-yuden disks burn at 8x in all 4 of my drives, the 20 or so I burned have no errors.

I also noticed the ty disks are read faster then the riteks. Perhaps a crc check makes the difference.

I noticed little black curved marks on my defective riteks.

The marks are towards the outer edge and are on the dye, deep inside.

Sort of like a little mark from a sharpie marker pen.

Guess the riteks aren't as they were. I have used the silver finish riteks and they were ok, just the printable disks are bad.