NEC 5500a and ritekG05

I have just install the drive in my laptop. It has firmware ver 1.5.1
Trying to burn some ritekG05 (labled 8x) disc, however the fastest speed I can get is 2x.
While I burn with ritekg04 (labled 4x), I can burn at 4x without problem.

Anybody has a solution to the problem?