Sony Dru510a

Hi wonder if you could help I have a SONY DRU510a dvd drive which I know does have a problem when burning DVDs @ 4x with certain media but will only actually burn @ 2.4x but then I found that if I used verbatim no probs would always burn @ 4x then recently after not burning any discs for a couple of months Ive now found that my drive only wants to burn verbatim discs @ 2.4x. I have updated the firmware, upgraded nero, tried sonys dss and uninstalled intel application accelerator to reanable the advanced tab on the IDE channel so I can select DMA but to no avail please help.