I've bought NEC 3520A about a fortnight ago, and recently payed attention to it acting strangely when reading DVD+R.
Used media is Verbatim Pastel DVD+R 4x written on 4x.
The main problem is that this disc read start (I use Total Commander for copy) is ok (6.5x it is), though at the beginning of the second file the speed drops at 1x, then somehow turns up back, and so on, skipping at the beginning of every file, but the "pause" is rather long.
Sometimes it doesn't come back to normal ever.
But the trick is if you start to copy a single file that was being read at 1x just before, it copies normally from beginning to end.
When I tried to copy files with Bad Copy the whole disc copied about 5 min (so it's ok with copy speed there), so I got really perplexed about the reasons of such strange behaviour in TC.
One more thing to mention is that when copy window loses the focus, speed goes down either.
PS Being new to DVD burning, is that graph ok?