Worse performance after upgrade to VPS


After upgraded to VPS (for more memory), the service is worse than before:

[]the A record of some domains are lost
]a drupal site in VPS performs worse than shared hosting (lots of blank pages)
[*]DH support does not reply in last 40 HOURS!

Any experience to share, when moving Drupal sites from sharing-hosting to VPS? Thanks

DH support please help!