It is not so easy to find out what the problem is.
I assume you use high quality media since you make your money with burning DVDs. Do you only have the LITE-ON DVDRW LH-20A1H as burner? I’d recommend a 2nd burner (the same or an other good ODD). It may be possible your burner has buning problems or your burner does not like your media.
Since you make your money with burning DVDs (maybe CDs?) you should buy a good scanning device. Unfortunately the good scanning drives are all vanished from the stores. There were BenQ’s, Liteon’s and Plextors as good quality scanning devices. I do not have an good advice for an “allround scanner”. If you have the possibility to get an PX-760A or PX-755A get it and you have a good scanner (and burner).
The problem with your Toshiba SD-M1502 could have two reasons. This DVD ROM is rather old. It should be from 2001 or 2002. It is maybe aged ad a little picky with burned media. I also have this drive (I also have SD-1712) and I do not have any problems with it.
I do not think that Nero is the problem. I guess the problem results from a poor burn quality. For this reason, tell us more about your used media. Do you use the latest firmware for your burner? Do you have a quality scan capable drive?