Google SiteMap -- Effective?

sitemap.xml in google webmaster tools

[nzap]

Hi, I recently implemented a google sitemap, through google webmaster tools using the correct protocol and file type.

But I noticed that one of our main listings went from #1 to #3-#4.

This was a listing that we had strongly as #1 for several years. It's fluctuated from time to time from #1 to #2 #3, etc. But over the past 12 months, it had been consistenly #1, until about 1 week after I implemented the sitemap.

Is this is a coicidence? Or did I cause this through sitemap?

One thing I know is that google was visiting my site every 3-4 days, I put "7" days in the sitemap field called "frequency". Not sure if I should put 3 days?

I only really change my web content on the home page once every few weeks at most, perhaps only once a month.

What happens if I simply delete the sitemap? Will that negatively effect our rankings? or will it simply put things back to the way it was before I added a sitemap?

Maybe it's something completely different going on.

Any ideas?

Thank you,
NZAP.