Default [Q] Bad bluetooth connection i9505 w/ 4.4.4 GPE

I have an S4 i9505 with GPE 4.4.4 and kt-kernel.

Now I also have a Plantronics Voyager Legend which is from my understanding a pretty high-quality headset.

However there seems to be a very bad connection between the phone and the headset. There's stuttering, loss of connection. Mostly during calls - music and video seems more fine.

The 3g signal is good, and there's no stuttering when using the phone without headset.

Are there any tweaks I can do or is this a common Android error which cannot be perfectly solved? I've googled and found known issues with connection on Nexus phones, but as far as I understood the known error was regarding pairing - and the pairing works fine.

Holding the phone in my hand near the headset reduces stutter. Having it in my right pocked, headset in left ear, makes the stutter worse.

phone: i9505(g)
Android: 4.4.4 GPE
Baseband: I9505XXUFNA1
Kernel: 3.4.94--KT-SGS4-KK4.4-TWGE-INTL-06.19.2014.ktoonsez #1