You can try burning some 8X DVD+R media to see if 8X can be achieved. Choose a manufacturer from the Plextor recommended media list and rated at 8X. This should help you determine whether the media is involved in your PX-708A being stuck at the 4X burn speed. Regardless of what the burning software may be reporting as the actual burn speed, so can use the actual burn durations to distinguish between 4X and 8X speeds.
Also, depending on what the media code is reported by the Memorex 4X DVD+R media (I suspect RICOHJPNR01, but possibly different), this media is not necessarily guaranteed to burn at 8X on the PX-708A. It depends on whether the media code is actually on the Plextor recommended media list, and even if so, the 8X burn speed is not always assured. Disc quality has been known to vary, even among discs within the same spindle. Here’s a relevant statement on Plextor’s website:
[B]The 4X DVD+R media provided to Plextor by these disc vendors, and listed on Plextor’s Recommended Media List, has proven in factory testing to be of consistently sufficient quality for the PX-708A/UF to write at 8X. However, the media manufacturing process is not under Plextor control, so Plextor cannot guarantee the quality of future batches of media from these vendors. Vendors may change dye formulation or the way they test and grade their media, resulting in various levels of disc quality throughout their distribution channel. Media quality can vary between different manufacturing lots, or even between discs in the same package or spindle. As a result, not all recommended 4X DVD+R media from any given vendor will write at 8X every time.
This does not reflect a drive problem, nor does it reflect a problem with the media. It does reflect the drive’s evaluation of the media and the determination by the drive firmware to reduce the recording speed to guarantee a quality recording. If your specific DVD+R media is not of sufficient quality to sustain 8X recording, the drive will then ensure a good disc burn by recording the disc at the vendor’s stated speed rating of 4X.[/B]
Plextor apparently favors burn quality over burn speed, so it is possible that PoweRec is lowering the burn speed to 4X (or refusing to allow 8X), usually due to burn quality issues. If you have PlexTools Professional you can determine what it reports as the maximum supported burn speed for your media.
If you can confirm that your PX-708A is operating properly in UDMA mode 2, then your PX-708A has either become more picky on media, or has suffered some kind of hardware degradation that affected the burn speed. Several months ago, my PX-708A suddenly stopped reading and burning all DVD media, and downgraded itself to a CD-RW burner, so I had to RMA it to get a replacement. If the drive hardware is deteriorating in some way, the symptoms can manifest themselves in various forms.