JUSTIN BIEBER loses it and we’re NOT talking about the projectile vomit he’s been spewing lately!  JUSTIN has gone ballistic over his sweetie Selena Gomez’s flirty friendship with her “Spring Breakers” co-star James Franco!

The mop-topped pop star, 18, threatened to deck the Oscar-nominated actor, 34, after seeing pictures of Selena, 20, cozying up to him, sources say.

“Justin is steaming at Selena for striking up what he considers an inappropriate relationship with James,” said a source. “But he’s directed most of his anger at James.”

The hunky “127 Hours” star has gotten under Justin’s skin ever since he got close to Selena while they filmed the indie drama earlier this year. And when photos showed Selena hanging on Franco during a “Spring Breakers” screening at the recent Venice Film Festival, Justin flipped out, insiders say.

The “Baby” singer was in Los Angeles rehearsing for his upcoming world tour, but “he called Selena and said that if he’d been there, he would have beaten up James for coming on to her,” said the source. “Selena told him they’re just friends and to stop acting like a spoiled child.”

Despite that, Justin surprised his girl in Toronto after that city’s film festival a few days later.

“Justin acted like he flew to Toronto to support Selena, but the truth is that he wanted to keep an eye on her,” said the insider.

As THE ENQUIRER previously reported, Justin’s mom Pattie Mallette fears he’s turning into a mini-me of his troubled former drug addict dad Jeremy, and insisted that he attend anger management classes.

“Justin is under enormous stress, so he’s acting out,” said the source. “But Selena has warned him that she’ll dump him if he throws another temper tantrum like this.”

But Bieber may blow another gasket after Selena flashed photographers while sunbathing in a bikini by a Miami pool on Sept. 17, reportedly spreading her legs salaciously after catching sight of them!