Peter,
the first 128mb CF card I bought for the D30 was a Transcend card. It let me take some pictures and then give me errors. I ended up with several corrupted (lines, half pictures, strange digital noise) pictures. I realized it was the card so I got another brand (Dane Elec) and that one worked just fine. I've tried Transcend again at 256mb and it works just fine.
So, corrupted files might be created by 'bad' media (cards) or maybe a defective read/write unit in your D30? Or you might have dropped/damaged/magnetized the card.
Just a few directions to search...
Bouke