OK I've narrowed it down to this. On my Mk2a w/2.0b7 on it runs fine at Home but when put in the Car is OK until I press the Rio-remote Source/Power button. Then the player locks up and trying to reboot it by pulling it out and reinserting into dock doesn't work, the boot screen stops at Starting Audio... and I can get no more out of the player until I reinstall software.
Update...Using the Rio remote & pressing Source/Power at Home invokes freezing of the player as well so it's not just in the Car as I thought originally.