Ritek is not the best media to use. In the past Ritek media was one of the better brands of media but that is NO MORE. I would suggest you use the RECOMMENDED BRANDS OF DVD MEDIA that PLEXTOR supports. RITEK is only on the COMPATIBLE list of PLEXTOR which doesn't mean the media will work correctly or successfully with the 708A drive.
Use MAXELL DVD-R 4x speed with media code TYG01 which is TAIYO YUDEN or media code MXL RG01 which both are MADE IN JAPAN. On the MAXELL SPINDLES as long as it says MADE IN JAPAN those are the ones you want to purchase. You could also use DVD+R 4x speed MAXELL which is manufactured by RICOH.
In my opinion it's the RITEK media that is causing the coasters. I used RITEK G04 media when I first began dvd burning but then their quality began to slip. I began getting coasters or parts of the dvd movies that I backed up would freeze up towards the end. Than I began using the MAXELL, RICOH & TAIYO YUDEN media and no more coasters or freezing up. Check out the PLEXTOR FORUM at cdfreaks.com