I had the same problem with my DVD-RW (Plextor PX-760A). And it was Verbatim DVD-R media, as well. Sometimes there were several speeds in the combo (up to 16x) and sometimes only 4x. Also, every second attempt to burn had been failing with disc calibration error (whatever that means). Then I upgraded drive's firmware from 1.03 to 1.07 and it seems that the problem went away.
There is a possibility that both drives (PX-760A and DVR-111D) have the same OEM vendor or use the same microchip. You can try following unofficial firmware: http://forum.rpc1.org/dl_firmware.php?download_id=2220. Cavea