I think I am a little bit closer to understanding these mysterious random drop-out issues.
My RS-220 stopped having audio signal interruptions after the only ancient ASUS WL-500g WiFi Access point was replaced with a newer Devolo dLAN 200 AV Wireless N.
The improvement is quite remarkable and I can now listen to music around the house for the very first time without a glitch.
So the conclusion is that certain misbehaving wireless access points will interfere with the RS-220 transmission path causing the infamous signal drop-outs. The house is currently covered by four Devolo N points and it has no effect on the audio signal quality.
Unfortunately if you are living in a block of apartments your options are very limited. But if you have control over the WiFi WAPs it is worth experimenting a bit. I have mine quad WAPs set to auto channel learning and they for some reason all vacated area around the central channel 6. I should also mention that the same spectrum is being occupied by 15 wireless alarm sensors so it is pretty packed.