Unable to Post in my blog after switching theme and revert back

I switched themes on a working wordpress blog, then I revert to the previous theme.

When I reversed, recent articles, comments, archives are not showing up in the front page and the website is loaded very slow in Firefox and only load the top of the page in IE.

So, I back up the site from an earlier version and everything seems to be fine, but every time I add a post and publish, the front page does not show the recent articles, comments, archives. Only the front page, all the other pages seems to work fine. And again, the front page loads very slow in Firefox and IE. When I remove the post I added, everything goes back to normal.

Can someone please help me find out what the problem is please. All the help will be greatly appreciated

Thanks