How many different brands and disks have you tried? Did you try any Verbatim/Taiyo Yuden/Made in Japan disks?
The 750 is in general, not that great of a drive but you should be able to get some decent results with the high quality media listed above. If not, then your drive is might be defective.
If you do plan on burning DL DVDs with the drive, The only disks Plextor recommends is Verbatim 8x DVD+DL (MKM 003).