IGO voices go loud every now and again

Hi All
Love these forums for help and advice...hope i got this in the correct section.

So now i'm hoping some one out there can help me as i have googled and read through lots of forums but cannot find a solution to the below.

I have tried several IGO GPS's of this site from IGO primo /2.4 to IGO nextgen and i seem to have a problem where when they are being used i get a second where the voice goes very loud (like its trying to blow my speakers) then it will run ok and do it again randomly, the same direction can be read out multiple times and only do it once has anyone had this fault and fixed it.

I have a no-named Chinese 2din unit brought from ebay running winCE6. (http://www.ebay.co.uk/itm/Headunit-7638CH-MAZDA3-7-Car-DVD-CD-Radio-GPS-Sat-Nav-Stereo-Bluetooth-DTV-IN-/111230919169?hash=item19e5e12201:g:F-sAAOSwstxVMNvD)

I have tried different voices including TTS ones
I have tried various SYS.texts some with volume limiters on some nearly blank and no sound level but i still get this odd bug.


Thanks in advance for help
CJ

Update:
Had the stereo replaced with the same model due to a different fault (crashing while using recording camera)..and i have still the same problem but..
the IGO supplied with it runs ok but cannot play music now as soon as it finishes the track its playing when gps started is stops playing any music.
so i still have same effects on opening post with every other IGO i have tried but the music plays (usb stick for info) so its either the software on stereo it or a strange hardware fault.
If anyone know how to keep the music playing on IGO Primo (version 9) that would get me up and running.

thanks in advance..
CJ