I managed to find the source of the stutter. When I bought the head unit it had a default Android launcher where I had to set the default navi program. At that time I was using another navi software so that was set as default. After so use I decided to replace the default Android launcher with another launcher the had a more OEM look. This launcher also required to set the default navi program. In the mean time I switched from the previous navi software to Igo but forgot to switch the default navi in the Android launcher. In short words, the Android launcher was fighting with the new launcher as to who controls the navi voice and thus came the stutter. As soon as I set the same navi software on both launchers the stutter went away and the voice instructions are perfectly clear.
I hope the above explanation helps others in the same situation.
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