Paulina Rubio's Ex Husband -- She's CRAZY!

Paulina Rubio needs to have her head examined because she's frickin' crazy ... so claims her ex- husband who is trying to get a judge to order a psych evaluation on her.

We broke the stories ... Paulina and Nicolás Vallejo-Nájera have been battling for more than a year over their son. It's ultra-bitter, with each accusing the other of alienating the boy from them.

Vallejo-Najera is now pulling out all the stops. He claims she hasn't undergone a psych exam as ordered by the judge, so he now wants hizzoner to make it clear -- she better get her head shrunk, or else.

BTW ... Vallejo-Najera says he handed the tyke over to the "X Factor" judge on August 3, and hasn't seen the kid since.