I've a strange problem with vista

Hello

I’ve got a TEAC in an USB 2.0. external unit, that is basically an OEM liteon. I’ve upgraded it’s firmware to 1693S. I’ve been burning DVDs for a year under XP without any problems.
After installing Vista I noticed that I got really crappy transfer rates from the unit. I installed Nero and ran DriveSpeed. I noticed that my unit was limited to 6X. I manually increased this to 8X, which is now the maximum for some reason. Under XP I could select up to 16X.

To sum up: My external 16X USB 2.0. DVD burner is limited to 8x for some reason. Device manager shows no errors, everything else is working perfectly.
Any ideas?

Which version of Vista and what bit (32 or 64)?