Drive Recognizes Some DVDs & NOT Others

i, repeatedly, tried to use the search feature, to no avail (just kept displaying blank page), so don’t trounce me if this has been resolved in another thread.
i did read through some with similar probs. (drive not recognizing ANY DVDs), but these did not apply to my situation.

my NEC 3530A has, periodically, given me problems over the last year by not recognizing certain types of media. For a while now, it seems to recog. CDs, CD-roms, and some commercial DVDs… but NOT others that are the same region, clean&scratch-free, and that work in my stand-alone dvd player (it does not give an error message…it acts as though there is no media, at all, in the drive). It also cannot read dvdr that i have succesfully recorded a film on (it used to read them fine), but instead reads them as blank media.

i have:
-checked the connection of cables
-uninstalled/reinstalled ide controllers & drive
-had the drive replaced (so i know it is not the hardware its self - it worked perfectly for awhile… then business as usual)

the only other thing i personally know to do (or have been able to find info on) is cleaning the optic lense with compressed air or drive cleaning disk. i am going to try this in a few days when my friend gets back in town and will let me use his - but i am doubting this sort of prob. could be caused by some dust.

what i’m working with:
dell dimension 8400
windows xp sp2
NEC 3530
more info needed?

any info/ideas/solutions/direction would be greatly appreciated

i should also say,… it is not a problem of media protection
i use anydvd & i have been able to watch/copy some protected commercial dvds with my drive

edit: i’ve also flashed my drive not too terribly long ago with what i believe is the latest firmware (think it was 104c)

i have an nec 6500a and 6750a running on tecra 8200 and 9000 computers.

i too have found that the read capabilities of the nec drives i own are to be generous not nearly as good as the write capabilites.

i can write + - dvd of average to tayo yuden qaulity with really great results.

the nec burns beautifully…
the read quality of much to be desired.

i have found for reading any all dvd media…commercial and home made i prefer to use almost any other dvd drive than the nec 6500 and 6750.
thye read slower and with more hiccups when the media has scratches or extensive use showing. for reading the nec drive i own are very fussy and temperamental.

the optics and or firmware is just not that good relative to their burn quality.

anyway just my opinion.