Hi,
I'm not sure if you've resolved this, but you can try repairing Office by going to Control Panel...Programs and Features...Uninstall a Program...look for Microsoft Office and click Repair.
If that doesn't work, then perhaps it's your ISP or email provider that limits how quickly you can send emails? If that's the case you might be better with
a tool that will throttle how fast your mail merge emails are sent.