Do I need a new Bluetooth headset?
If I recall correctly, for Navigation directions over BT you need A2DP (just like music).  I just checked the specs on Plantronics' website for the Discovery 975 and it doesn't mention having this.

There's also a setting somewhere in Navigation to route it over the speaker or the headset; I remember turning it off on mine because it was interrupting conversations when I was trying to listen.  Unfortunately I can't recall exactly where the setting is; I'll update this later if I find it again.