Check the specifications at Plextor 708A. Your Plextor is an 8x drive, but only with +R DVD media. -R media can be burned only at 4x max. This was common in earlier drives.
Concerning burning only at 1x, this is usually an UDMA issue. UDMA 5 has to be the transfer protocol for a hard drive, as the Plextor has a max rate of UDMA 2 (33 MB/sec. burst). You should check your settings in the system BIOS to make sure that Ultra DMA is enabled for the drive. You didn't say what operating system you are using, but check in Device Manager also to confirm the Ultra DMA is enabled. If you are using WinXP and UDMA is not enabled for drive, uninstalling the drive in Device Manager and rebooting usually will reset the top transfer rate if the BIOS is set properly. The drive will be reinstalled after rebooting, and you can check Device Manager to confirm the speed.
If using Win98SE, make sure the DMA box is checked in Device Manager.