Intermediate site not accessible after IP change

I’m really lost on this issue and would like to hear from experts on possible root causes. I have a site running WP 3.5.2 with WP maintenance mode plugin turned on. Everything was working fine until I applied for dedicated IP (for SSL) 3 days ago. Since then I’m encountering intermediate site not accessible issue.

Basically two situations happened, one, when trying to access my site’s URL/IP, the browser shows “default server page (description in the page says IP issue)”. And two, even after I got through and start working on the site, a “404 not found” would show up randomly. For both situations, I’ll have to wait for a while, clear browser history/cache then cross my finger that next time would be a pass.

I have contacted my host many times and they have verified DNS is fully propagated. And I have followed their suggestions below but problem remain:
1) Wait for 24 hours – it’s been 3+ days!
2) Clear local DNS cache
3) Clear browser cache/history

I’m wondering if any WordPress related setup/file could possibly cause this situation? Or it’s purely my host’s issue.

I have not been able to work consistently due to the issue. Any suggestion/help would be greatly appreciated.

Wayne