I'm having problems with DVD quality

Hi folks
I’m having problems with DVD quality. Bought a LDW411-S and burned various DVD brands(RICOHJPN01(+R 2.4X), RITEKG03 (-R 2X) and CMC MAG F01 (+R 4X)). That was giving me 1 coaster at every 3 burned disks and the other 2 were bad quality ones(high PI and PIFs). Tried to burn with DVDDecrypter, CopytoDVD, Nero 6 and no change. After I`ve tried a lot of things, I changed my burner for a LG GSA4120B but ÃŒ’m still burning bad quality DVDs, especially in the last tenth, were PI reaches 300 and PIF reaches 35(average).
DMA is activated and working fine, have used a lot of diferent ASPI drivers, cleaned burner lens and so on, burned at 1X…still bad quality.
I’m thinking if that could be my system…I have a PIII650-AsusP3V4X-256MB of RAM(PC133)-5400-20GB Maxtor HD and ÃŒ’m running Win2K server. During the burn process, when I look in task manager, CPU usage is about 4% and there are about 50MB of free physical memory. The fragmentation of the HD is a little high because it’s a small HD and I can’t save a 4,36GB image file in it without having fragmentation. But during the burn process the read buffer and the device buffer are always in 97/100%.
Anyone can help me?
Thankyou and sorry for my english.