ND-3500 and ND3520 can't read burned DVD-R

I have a ND-3520 and one of my work mates have a ND-3500.
We found that for some strange reason our drives can’t read burned DVD-R discs.
When a burn of a DVD-R disc finishes, the disc is still fully readable, I can copy files from it and run a transfer rate test, but once I eject the disc it won’t sync in the drive any more. In Windows Explorer is shows as an unreadable cdrom (blank?). This problem does not exist with DVD+R.
Between us we’ve tried many media brands, but the problem is consistent.
The strange thing is that for both of us it once worked perfectly ok, and suddenly it started.
The drive won’t read DVD-R discs it burned itself, or in other drives. All these discs are perfectly readable in other drives. All the discs I tried are single session.

I’ve tried the 3 latest official NEC firmwares for my ND-3520, but none solves this problem.

My system:
Windows XP Pro SP2
Asus P4C800 Deluxe (firmware 1019)
P4 3.2GHz (Northwood)
Corsair 2 x 512MB DDR400
Adaptec 39160
Seagate Cheetah 15k.2 73GB x 2
Quantum Atlas 10k II 36GB
ATI Radeon 9800 Pro 128MB
NEC ND-3520 (firmware v3.07)

Any ideas?

With the 3520 I and others have had same problem as well as some 3550 users. Notice the 3540 review rates it below average for reading, and evidence indicates that’s true of other NEC drives through 3550 at least. I have Pioneer 111 and 107 neither of which have this problem, and review of the 111 has it rated an excellent reader. Some NEC users don’t seem to have the problem which I can’t explain, but for those that do it may be necessarry to stick to +R media.