Your results in data transfer with data CD’s are normal for this drive. The DMA Multiword 2 bandwidth of 16.7 MB/sec. is adequate for full performance in CD reading, which requires (52 x 172 KB/sec [raw]) only 8.9 MB/sec for a 52 speed reader.
This is not the same with DVD reading, which spins 3x faster than CD at the same speed rating. At 1x speed, a DVD transfers about 1,108 KB/sec. At 16x, a DVD will require 17.7 MB of bandwidth. This exceeds the capability of DMA Multiword 2, and explains the cause for the drop in performance.
At 10.3x the DVD is using about 11.4 MB/sec of bandwidth, and although this does not approach the theoretical capacity of DMA MW 2, it is probably hitting the practical capacity in your system.
WinXP has a known bug in that is sometimes spontaneously resets ATAPI device connections from UDMA to PIO mode. There is no known fix at present but there is a workaround. This involves going to Device Manager and removing the IDE channel to which the PIO device(s) are connected and rebooting. Upon rebooting the IDE will be detected and the devices will be reinstalled. Usually this works to redetect and restore the UDMA capability of the ATAPI devices.
Your problem does follow this pattern exactly, but it is similar. It is possible that this bug in WinXP has reset your drive connections to DMA 2 from UDMA 2 (33.3 MB/sec). I would suggest trying the fix above by removing the secondary IDE channel.
Let us know if this works.