Upgraded 2100 writes/reads bad !

Hi. This is my first post and I came here because I'm having many many troubles with the NEC DVD recorder 2100.
I had to upgrade it to burn -R disks, and I'm not too concerned about DL.

I tried both 2500 and 2510 model transformations... BUT ! :(
I'm thrashing most of the dvds I burn, because I can't read them even the burning process (both 2x or 4x) terminates correctly and the disks are perfect from many brands (verbatim, princo, memorex, fortis, bulk).
O/S: WinXP Pro SP2 - Soft: Nero 6 or Alcohol

9 times out of 10 I have reading problems as soon as I close the disk (even during verification), then the disk seems containing files, but most of them are not readable.

Well, I have a "phisical object" reported (it was...) by WinXP :
_NEC DVD+RW ND-2100AD
bundled in a Fujitsu-siemens E600 desktop PC

With different firmware upgrades (from TDB site), I came to 2510 thanks to the 2510RPC1.BIN. I even tried some other fw to have a 2500, but problems are still the same. With the bin above, the O/S reports:
_NEC DVD_RW ND-2510A

I think the problem is due to the fujitsu-siemens bundled version of that burner, but a colleague of mine has an identical PC with the same RPC1 fw upgrade and he hadn't a single problem!

I even had my dvd recorder replaced from factory (of course after restoring the original 2100 fw I backupped ... FYI: it is possible - and easy - to do that!!!).
Note: when I restored the 2100 fw, the drive became an ND-2100A loosing the trailing 'D' !!!

But after upgrading again to 2510(rpc1), I'm still having the same frequent problems: burning phase is fine, but the disk becomes unusable.

Flaws when writing the TOC ?

I even tried to reinstall the OS from scratch, but I'm still experiencing those problems even with only XP SP2 and Nero installed (with all the drivers, of course).

Could you help me with some tryout I could do yet ?
It is destroying me !!!

Thanks