Cockatiel, obsessed with my girlfriend!

Hello all,
   
I've had my pet cockatiel, Bobo, for somewhere between 15 and 20 years now. He's always been a good little bird, and loves to sing (his own variations on many sounds). He was hand raised and initially had his wings clipped. I've since let his wings grow out so he can get around and at one point was even so tame that you could pretty much call him to fly to you.
   
Over the last year or two, which is about how long I've been with my girlfriend, he has literally become obsessed with her. If he hears her voice or sees her, he becomes extremely agitated and will shriek loudly if she leaves the room or is too far away. It's extremely stressful for my girlfriend knowing that when she wakes up, she can't make a sound or Bobo will start having a fit.
   
Bobo has never been like this before. What could have caused him to start this behavior? And more importantly, how can I correct this? Any help would be greatly appreciated .. we're on the verge of finding my little buddy a new home to reduce the stress level in our household.

Hello I havn't been on for a while and I saw this .... Sorry if you already have delt with the situation but maybe give it distractions like tree branches to chew on, toys etc. But not flooding the cage cage with toys.So hope this helps if you still have your bird.