It's normal fluctuations for parking...every visitor is different, it also has to do with what advertisers are showing up....sometimes if what they are looking for is not there advertiser wise then they just won't click...also make sure your page has not re-optimized with new keywords based on user searches..if it has you can just remove new keywords easily in your control panel as I'm sure you know. Pay attention to the traffic analysis page and keep track of who is doing what click wise.