Hi all im new to seo but i have noticed recently with a couple of different sites of mine are jumping from say page 2 to page 7 ,i could do the same search for the same keywords seconds apart with different results.
Is this down to bad seo on my part or just google.

There are a whole array of things that can effect your placement of search rankings. Just the fact that pages spring up on a regular basis can cause the rankings to change. Companies poor millions of dollars into their SEO and still may never see a front page listing.

Keep in mind that Google's pagerank mechanism is said to take into account over 700+ variables. The best thing you can do is focus on your content, keep it on target with your keywords and forge alliances with other sites in your industry. The idea is that you want to tie your keywords and content to your message and get people to your site as well as keeping them coming back. The returned visitors and an on topic message with your business will drive your search results. But keep in mind that even if you do get to number 1 page or even number 1 result for certain keywords, you are never going to be able to hold it indefinitely. Expect your results to go up and down. It can just as quickly go back up next week.

Besides, you may not be tops in a certain keyword, you may be number one in a keyword that others can think of. Just don't beat yourself up about the placement and focus on making your page as search friendly as possible.