I have observed much the same with a retail A09. It arrived with v1.13 firmware which I immediately flashed to v1.17.
I originally tested the drive in a Prolific P3507 based external enclosure via USB and found the read speed also plummeted at about the midway point. It would eventually hover around 1x - 2x until making it to the end of the disc.
This happened with discs of type DVD-R, DVD+RW and single layer retail DVD (DVD-5).
The DVD-R discs I tested were 4x (TYG01) and 8x (TYG02).
The DVD+RW was a 4x TDK (RICOHJPN-W11-01).
As I have the retail A09 drive, I was also able to test the different read speeds offered by the Quiet Drive utility. It made no difference unless I used Quiet mode with the read speed limited to 8x.
Performing a 'Create Data Disc' in Nero CD-DVD Speed to test the writing ability resulted in the standard 6x, 8x, 12x Z-CLV graduated burn in about 7:10 minutes.
So, thinking the reading problem was associated with the external enclosure, I installed the drive as Master (by itself) on the Secondary IDE channel in a Intel 845 based PC.
Using a 80 wire IDE cable, the drive was reported as being in Ultra DMA Mode 4.
I repeated the tests and found the results were almost the same as when the drive was in the external enclosure.
I decided to wait for a new firmware release. Soon enough, v1.40 appeared on the Pioneer Europe site.
Testing v1.40 in the external enclosure gave similar results to the v1.17 firmware.
Results on the Intel 845 based PC were more mixed.
The drive seems to be useable (as a reader) with this firmware if I set it to Performance mode but with the read speed limited to 8x.
I also tried using a 40 wire IDE cable so the drive was in Ultra DMA Mode 2. It exhibited the same strange reading behaviour unless I restricted the read speed.
At the moment, the drive is only useable in the Intel 845 based PC with the read speed limited.
It does seem that there may be some issues with the reading abilities of this drive for a very small number of users. Is it perhaps a combination of the IDE interface and certain motherboard chipsets?