S5 Bluetooth and speaker problems during call?

[glennn Armstrong]

I am having a problem with my S5, during a call it keeps jumping back and forth from speakerphone to handset with out me touching anything and the same when connected to my in-car Bluetooth it connects automatically when i get in but then when i make a call i can only hear the sound through the handset and have to press the Bluetooth button twice to get it to go through the car stereo but then all of a sudden it goes back to just coming from the handset, any suggestions as to why this is happening and how i can fix it?

Thanks.