I got the impression that suggestions are always given, no matter which type of error is being flagged. See
http://office.microsoft.com/en-us/he...584.aspx?CTT=1 (Office Hours: The next generation of the spell checker). But my native language doesn't support the contextual spelling & grammar feature, so I have no practical experience with this.
In Office button | Word Options | Proofing, you can choose not to check grammar as you type; see if turning off the option for the contextual spelling makes a difference.