problem with website

I have been having a problem with my site for a few months and here is what is happening. My site normally shows up in google search results around page 15 to 18 and has been there for at least 6 months. The site is about 2 years old and didn't move from this area even after all the google updates. However for the past few months whenever I add a new page to my site the site vanishes or goes to about page 75 or so in search results. I asked the people on the google web info center and they tell me that it is because I have hidden links on the site. The hidden links are in the drop down menu and are visible when you hover your mouse over the main site tab just like other sites do. The problem with there explanation is if it really were the hidden links then why does the site always end up back around page 15 or so in search a about a week later. If it were the hidden links wouldn't the site stay way back around page 75 which is where it goes after adding a page. Anyone have any ideas what could be going on. Thanks Mark