I may be trying to teach yer to suck eggs but I would delete your itunes playlists and then redo them making sure of the following
1) Don't make more than 10 playlists (including playlist 1 - this is used for setting up the playlists properly)
2) Use short names for playlists in case of problems (shouldn't really be any since the ipod/phone will have done all converting and only thing is the sound output going to the grom)
3) Playlist 1 should only contain 1 track, any sized track it doesn't matter
4) If you disconnect your ipod/phone when leaving the car, make sure you stop it first, pause the music, turn off the car stereo in that order. When re-connecting use the reverse
5) try and make sure that playlist 2 is currently playing or paused before you turn the car stereo back on after reconnecting the car, this seems to work better for advanced mode
6) If any of your songs sound low in volume, turn off the stereo whilst the ipod/phone is playing and turn back on, go to any playlist other than 1, should then be ok
7) Check all electrical connections (seems obvious lol)
Thats all I can think of at the mo, my ipod 6th gen plays fine, only have problems if I disconnect the player when both units are still on
hope this helps, try the above and if you still need help I'll try and help yer out more m8