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

[638]

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.

[CmsBuffet.com]

the search for indexed pages is not the most accurate. it varies from time to time.

from my experience the number you see in Google webmaster tools is closer to the truth.

since you mentioned that your current system is ?view=x i would suggest switching to a more SEO friendly CMS.
just make sure the CMS has a solution for your current "SEO Juice".