i had a major drop with one of my websites recently. i was constantly moving up in the serps for some highly competitive keywords, I made it to the top of the 2nd page. Then I just started constantly going down. Now i can't even find my site for my main keyword. But it is still indexed and shows up great for some secondary keywords? I'm going to give it some time and hope it comes back. the site is about 10 months old, i was hoping i was past all the sand box issues
Check my post over here Google change its PR validation system and cache, Yahoo in SERP and yesterday MSN live was change too. I just update my tool to check SERP and PR validation.