i use sitemaps and sometimes i see a drop for keyword ranking and after a while it improves. I think it is all about SEO / competition and in between google PR updates.
Sometimes a deleted but important line of text on your homepage can have an affect on rankings. Check backups.
Hosting loc. for better spec. rankings ? No not that i know of.
We uploaded an xml site map on webmaster tools of about 65 odd pages some time back and recently added about 30 more pages to the site. 3 days ago, we re-submitted the site map with all 95 pages but for some reason we have dropped on the SERPS.
Google has to “slurp” these before you see improvement happended to me several times. 30 pages is a lot in a short time, do it gradually next time.
I could be wrong but too much content at once is not that good if the ratio is high between new/old content 30/65.
R.