Plextor 708A play problem

About half of the rented dvds play on my computer but they all play on the three stand alone players I have. I have most recent firmware installed. Thoroughly cleaned the dvds. Amber light flashes twice-unable to read. Region code 1 setting.
Any suggestions? Thanks.

do the self diagnostics test on the drive and if it fails then RMA it back to Plextor. the instructions for the self test is in the manual.