What could have happened?

Ranking stable for years

[ffctas]

Over the weekend many of my internal category pages took a huge hit in g ranking. These pages have been in the top 10 for at least the last 3 years.

My site is PR5 and gets crawled daily. We did change our programing language from coldfusion to .NET over a month ago. We did all 301's correctly and this seems to have worked fine.

It's weird, some pages have not changed and others have changed dramatically.

Any ideas?
Thanks