Nec 3520 Burning Problem

Im using an nec 3520 flashed with 1.u6 f/ware;with ritek go5 discs i can burn a 12x no problem,ive just purchased some ridisc or ridata 8x silver printables with CMC MAG AE1 dye which with this f/ware can be burnt a 12x but unless i set burn speed at 4x it kicks the disc out about 25% through the burning process! Anybody got any idea what the problem might be?

Poor quality disc ?

its possible i suppose,anyway these are supposed to be quite a good disc or so im led to believe! maybe the information i was given is incorrect?

Just updated to 1.u9 f/ware and now if i burn a 4x it kicks out but seems to burn ok at 12x!!! somebody please help