jimbeetle - 4:40 pm on Dec 19, 2008 (gmt 0)
whilst the original lesser competitive and sometimes 'long-tail' keywords are still showing too.
You must be losing trafic from somewhere. Have you identified where?
You give a hint in what you wrote and Shaddows is right on here. It might be worth it to run some before and after numbers to see if those terms that are "sometimes" still appearing are the underlying cause of what you are seeing.