Cat has suddenly gone psycho and attacks my GF

My GF and I have had our male cat for 5 years since he was a few months old.  He is a fixed American Shorthair.  A month ago, he became extremely aggressive toward her.  Half the time he's fine and sits near her, allows her to pet him and give him treats.  The other half of the time, usually beginning in the morning when I'm gone at work, he becomes viscous and aggressive toward her, hissing, screaming and growling. He stalks her through the apartment and screams at her, hackles raised and will lunge at her with teeth or claws if she gets too close. This generally lasts the entire day.  Sometimes two.  He will chill out in one of his spots and then if she comes into view, he goes insane again.  Its directed at her for certain.  She doesn't hit him or anything.  She's an insane cat person.  Total crazy cat lady.  We have not changed residence, moved or changed his food.  His water is in the same place.  His litter is scooped regularly.  No person or pets have been introduced into the household. I can't take much more.

Something has changed.

It could be a new scent... has your GF started using any kind of new scents?  Or maybe another cat is leaving their scent outside?

It could be medical.  Have you taken him to the vet?

I don't think so. There could be other cats scenting the porch outside during the day, as we live in a neighborhood with a lot of strays.  And we took him to the vet for a check up but he was fine.   I'm clueless.  I think if he was sicker in pain the aggression would be directed at both of us equally.  He's always been more partial to me (and guys in general) but now he just hates her.

I just did an experiment.  I went in and pet him for a few minutes (he's chilling out in our bedroom with the door shut.  When he saw her feet under the door he went nuts.  I left the room, shut the door and went to a part of the apartment where he couldn't see me.  A few minutes later, I walked past the room where he could see MY feet under the door.  Nothing.  She goes by and he freaks again.  

can you try getting more of your scent on your girlfriend? maybe she could wear your t-shirt after you wear it. 

This may sound odd...but maybe your gf should go to the doctors? Something has changed...is she pregnant?

Is she wearing shoes as she walks past or barefoot. Have her try stomping her feet as she walks. Men usually walk louder.

Feline hyperasthesia comes to mind as well.

As for smells is your gfs on birth control? Has she changed any medications lately?

What about changes in perfume or deodorant? Shampoo?

Very strange. Is it possible she accidentally stepped on the cat or bumped the cat with the door?