MacOS – spaces switching unexpectedly on osx yosemite

To isolate a DNS issue, you might try using google DNS by setting your Mac to use 8.8.8.8 instead of the DNS server it assigned or received from your router.

From there you can use nslookup and ping to test network connectivity and DNS resolution.

Note: This is a blind answer attempt: I don't have Microsoft Office on any of my computers.

Could you try:

System Preferences
    Mission Control

to deactivate:

    ☐ Automatically rearrange Spaces based on most recent use
    ☐ When switching to an application, switch to a space with open
               windows for the application

I deactivated both because they were always disturbing my working environment in a way I wasn't able to go ahead with what I was running in parallel on many spaces. Since I made this setting I'm able to work with an environment not running under my feet.