FaceTime from OSX (MacBook) frequently cuts out my voice for the listener

Question

FaceTime calls from my MacBook (OSX 10.9.4) to an iPad / iPhone frequently drops out my microphone audio for the other party, they report that the video is still fine when this happens (suggesting it's not a network issue) and that the audio for them keeps dropping, yet I can hear and see them fine.  It's really frustrating when they keep looking at me blank and asking me to repeat myself.
I'm calling from Spain back to the UK, I have a 100mb/10mb fibre connection and the other side has 24Mb/1Mb ADSL. I previously had 10Mb ADSL and had exactly the same problem.
The problem does not occur when I call them using my iPhone strangely, suggesting a problem on the MacBook.
On the last call I ran the Quicktime recorder during the conversation, the other side of the call reported the same audio drop outs, after the call I reviewed the recordings which were perfectly clear with no drops, proving the microphone of the MacBook is fine (which Apple recently replaced as they believed that to be the problem).
So points to the FaceTime software perhaps for OSX which there are very little settings for?  Has anyone else had this issue and have a fix?
I've tried turning off the 'Use ambient noise reduction' flag in Settings > Sound > Input in case that was playing some part in it but made no difference.

Answer 1

Have also tried using Skype instead and didn't have a single issue.
There are some similar threads below, but they've become so huge that different people have started different issues but there are a few posts within which seem to match the behaviour reported above, obviously have not read through all 18 pages there are so far:
Sound issues using FaceTime on Mac!!
However the one below, with the post from RobJames.com seems to be the closest, also some users above have said they have the same question too, so I'm glad I'm not the only one with the problem:
Re: Facetime audio drops
Since the last post I have reinstalled the OS on the MacBook and before installing anything else or restoring anything I tried FaceTime and experienced the same poor audio.  I don't think I can do anything more with the MacBook now as it's had it's microphone replaced and also OS reinstall. 
I've had a support case open with Apple since January this year, getting nowhere fast, they want me to try calling to other users but I've tried multiple devices at the other end (3x iPhones / 3x iPads) and they've also replaced their router.  I've tried calling them while they're connected on 3G and experienced the same audio issues but it's difficult to tell if this was due to using the 3G connection as the video was also stalling there and that's not what usually happens when we're both connected normally to fibre / ADSL.