By any chance, are the songs in the WMA format?

And, by any chance, are the songs recorded at 190kbps instead of 160kbps? (If I recall correctly, 160 is the default, and 190 is the high setting. Or something like that.)

A similar problem happened on the empeg-car player. It turns out that the WMA decoder for the ARM chip (which is a binary file- no source code- supplied by Microsoft themselves) had a bug which prevented high-bitrate WMA files from playing properly, and you got pops and such in the files.

If this is what's happening on the Rio Receiver (someone from Cambridge would have to verify if it is), then the only solution is to re-rip those CDs as either MP3 or as lower-bitrate WMAs.