I have a UK red 5800 - product code 0575581 on firmware v11.0.009.
When I connect it to my CK-7W car kit I get an intermittent problem whereby the sound through the car stereo speakers cuts in and out during a call. It basically mutes every second or so for about half a second, making the other caller's words pretty much unintelligible.
I have tried resetting the car kit, de-pairing and re-pairing the 5800 andtweaking the volume in case the mic is picking up background noise, but nothing seems to work.
On the odd occasion the kit will work fine, on other occasions it will start out fine then the problem starts again. Before I got my 5800 I was using the kit with an N95 and NEVER had the problem before.
Any ideas?, as I'm rapidly going bald from all the hair I'm pulling out!!!
I have exactly the same problem with my (German version of) the 5800 paired with my CK-7W carkit. I'm sure it's being caused by the phone since I tested it with my first smartphone, nokia 7610 from 2003 and it even gives a much better sound quality. Like you said, often you are trying to tell something and there goes the sound... Very very annoying...
I hope the software update I am about to do (to 20.xx) will help.
I refuse to buy a new carkit if the problem persists...