I wish we could blame it on SP2... or that storm of "cosmic rays" we had a few days ago, when it started acting up :) But I'm afraid it's a Nasm bug.
The "-g" switch never did work 100% correctly. One bug report we had said it made tlink (and if it linked, TD) "act like a barral of sexed-up monkees" :) Alexei Frounze put some time into it a while ago, and improved it considerably. From what you're seeing, it apparently still isn't right.
You're right that a beginner shouldn't have to investigate this problem... but nobody else uses TD :) If you *can* notice what causes files to go from being "lucky" to being "bad object"s, a report might help us track it down (no guarantees!) If you're able to isolate anything specific, post it here, or to the nasm-devel list, or file a bug report in the tracker, or mail it to me - fbkotler at comcast dot net.
TD with debug info is a *lot* nicer than TD without debug info, so it would be nice to get it working properly.
Best,
Frank