after trying a few ideas here, what has been "working" for me as a fix was
1- as recommended, I removed all my music by deleteing from the iPhone (4S, recent replacement for stolen phone, in case this makes a difference.) the data attached to music;
--unchecking sync music in iTunes, allowing it to sync w delete (where it keeps only iTunes purchased music)
--resyncing when rechecking the Sync Music -- for some reason I had to do this twice, after a reboot of iTunes & iPhone, but then it worked.
2- each time I modify music, i have to uncheck, Sync to nothing, then recheck and Sync again for my music changes to be applied. No reboot has been necessary the last two times I added music.
APPLE, it would be great to not have to jury rig a solution. Please fix!