What firmware are you using,I recommend Nil's 8x4all,12x rip. Different media has slightly different write strategies Nil picks the best from test results(e.g. burns different media with different strategies and uses the best one).
I've heard that for Pioneer A05 and A06 drives, after disc is inserted into the drive, the drive will test the disc and come up with the best strategy for this disc. Such function is too difficult to be devised for 8x recording. Hence, for A07 Pioneer chose to follow other makers such like NEC and Sony. The way NEC and Sony do is to test different brands of media in their companies and pre-set the writing strategy in the drive's firmware according to each manufacture ID.
Since Pioneer not many 8x media has passed Pioneer's test yet, the A07 does not allow many 8x discs to be used at its max speed. Nil picked the best writing strategy, but it may be just one strategy and would not be the best for all brands of media.
Still, I think Nil did something which is very very difficult and time consuming. There are about 38 parameters just in 8x writing strategy (6x has less parameters). Well, lots of them are set to be 00. But still, there are about 15 parameters that seems to be important. Each of these parameters might have 10 different possible values. How could Nil come up with writing strategy in such short time. Rumors says that CMC used 1 team of skill engineers and spent 1 month of time to be able to pick a good set of parameters for their 8x and 6x writing strategy!