Iphone 4 back-up freezing Itunes

So whenever I try to sync new songs to my iPhone 4, the back-up seems to take an incredibly long time, an when it finally gets to literally less than a quarter remaining iTunes just stops responding. Then as soon as I remove my phone from the computer it returns to running normally. it use to be a real chore to get songs onto my phone (having to leave it plugged in over night) but it was doable nownot at all..
also the few times it does get by the Back-up, it simply skips over syncing the new songs i wish to put on my phone an completes everything else.. yet according to itunes it states I havent backed up my phone in over 4 months? 0.o
Someone please help! I just want itunes to work an install my ******* music.. dont even give two ***** about backing it up but that seems to be what is always stopping me from syncing said songs.