Lost Keyword Ranking

i've completely lost one of my keyword rankings which was #2 for its search results on google. my other 2 keywords however haven't changed. any ideas why this happened?

i was recently infected with somekind of spyware code in my index page but i just cleaned it. google may have took me off its listings because of this but if it did, how long do i have to wait to be relisted for this keyword?

nevermind, its back up. werid. dunno how long it was unranked for. my site is also now malware free, hopefully.

it's unranked again! any ideas why?

one of my word density is at 14.38%, could this be way? if so, my other keywords using this word are still being ranked.

Minor variations like this are normal, wait and see if it keeps happening over the next few weeks. What period has this been happening over so far?

As for your KWD, whilst I don't believe that there is an optimal KWD, I do believe that there must be a level over which (and in conjunction perhaps with other signals) you may look spammy to Google. The way to test it is to get someone not familiar with search engines to read it and if they raise their eyebrows... then you've probably got too many instances of your keyword phrase in there, just write for your users.