goodroi - 2:30 pm on Dec 10, 2010 (gmt 0)
you can use an online sitemap validator. it will identify which line has an issue. after you fix it re-validate it to make sure you dont have multiple issues.
you may want to also break your big sitemap into smaller files. i doubt the size is causing problems but it wont hurt to do this. also it is easier to trouble shoot smaller sitemaps.