I've done plenty of link tests over the last 6 months, depending on the sources I've seen a few observations:
- Ranking stick
- You trip some sort of threshold (assuming anchor text) that makes your keywords bounce in & out by hundreds of positions at a time.
- You trip a further threshold which buries the keywords down hundreds of positions.
Maybe use OSE to run an anchor text report to look at the diversity of anchor text on your affected pages - that might show some extra data points.