Two of my sites were #2 yesterday for a competitive keyword .. Today , they both are more than #700 !!! .. I dnt understand this , am i sandboxed ?
As an addendum to your other sandbox post new sites generally rank well for any phrase they are targetting (even competitive ones) for a short period, before ranking very badly. After some kind of probationary period (a year?) they seem to start ranking more highly.
The age of your site? Google sandboxed new domain / websites.. I think minimum of 6 months.. this is to fight spam sites to get a higher rank.
You had a brief 'high' because of the keywords, this often happens, it does not mean that your site had something and then lost it. My advice is don't panic with every little change. Its proverbial swings and roundabouts.