[SEO] Why do some parts of my website disappear from GOOGLE and how can I stop it from happening?

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I have a lot of products on my website. Most of the URLs are like this: http://www.domain.com/product.php?view=x

Sometimes if I search all of the pages in my site on Google with site:www.domain.com I notice that from time to time pages disappear from Google even though they've been indexed before. Sometimes I have 100 pages indexed in Google and sometimes I have a 3000.

How can I prevent this from happening? I may be loosing a lot of potential clients that are looking for the products I am selling but they do not show up on Google.
I am not using a CMS, the application is entirely made by me, so I would like to know some tips to optimize it for search engines.