iPhone 6 sync problems with iTunes 12

I'm having issues with iPhone6 and iTunes 12 trying to sync music. I have about 6500 music tracks in my library which is probably bout 60/40 CD import/iTunes store purchase. Every time I sync my iPhone 6 to update added music, the new music will not add, and the iPhone 6 will dump album art that was previously there. The dumped album artwork includes BOTH types of files (CD import AND iTunes purchase) I have iTunes 64 (12.1.2.27) on a Dell Windows 8 machine and a 128G iPhone 6. After syncing all music, I still have about 60G available on the phone. On iTunes, I have "sync only checked music", but I have EVERYTHING checked (if i simply check sync all music, all music will not sync). The only way I found to add new music is to reset the iPhone and set up as a new phone which is ridiculous to do just to add a new playlist or new music. Kind of at a loss for what to do here here. Any help would be appreciated. On another note (may or may not be related), I notice iTunes stops responding while performing a function. Let's say, for example, I am downloading a purchased movie through iTunes to my hard drive, iTunes ceases to work. Task manager shows iTunes as not responding, but if I allow the file to complete, iTunes then works fine without a re-start. The Dell is brand new with 8G RAM, but memory is not a problem, and I don't have any useless programs running in the background. Any help would be great, Thanks in advance!