DVD-RW SD-R5112 Toshiba Writing Problem

Hi
Recently I’ve got that nasty problem I cannot solve.

Some of the DVD-R I write cannot be read at all.
Recently I’ve bought 10 DVD-R Verbatim 4x Printable (4087 507 -R B C 02172 written in the inner part (for example)).
One of them couldn’t be read, though 9 were completely OK.
(All 10 were written on 4x).
Moreover, Nero (I use version 6.3.1.6) checked the disk with no error, it just wasn’t recognized in TC or was recognized blanc in Explorer.
Nero sees no sessions on this disk and also conciders it blanc.
There are no visual defects on the surface and no seen writing defects (all the surface burned).
DVD-drive operates in DMA (as shown in system->device manager), but on the startup it’s recognized as pio 4.0… that fact bothers me.
DVD firmware is updated to 1033 with all limitations (I’ve made a degrade from
no limitations though).
The drive was bought a year ago. (Though the warranty time has already gone =()
WinXP SP2.

So what can be the problem?

Atmc,

Have used the firmware update from Toshiba’s site (http://www.toshiba-europe.com/storage/Index.asp?page=PFW&nav=RSC_SDR)?