Despite the couple appearing to still be together, Ryan Gosling and Eva Mendes' relationship could still be on the rocks.

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Sources claim the couple's relationship is not stable, and Gosling ex-girlfriend and former co-star Rachel McAdams could be the reason why.

"Their romance is definitely on shaky ground," a close insider reportedly told OK Magazine, adding that Gosling has been refusing to commit more seriously to Mendes.

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The source continued to explain that while the topic of marriage has often come up during the couple's relationship, Gosling has continued to balk at the idea of popping the question.

"For the past 18 months, they've been talking about taking the next step, to the point that on every vacation, Eva's been convinced he would be popping the question," the source said "The trip to Iceland in September was the last straw. She pretty much gave him an ultimatum, but he balked and she stormed out and flew back to the states without him."

Adding to the couple's potential problems is Gosling's recently getting back in touch with McAdams, who has reportedly been trying to win him back since splitting with Michael Sheen last year.

"Rachel has been very low-key about it all," the source said. "She's played it very smart and used the whole 'just friends' angle to get close to Ryan again, but she 100 percent wants him back. He's not the cheating type, but he hasn't been honest with Eva about how much contact he has with Rachel."

Gosling's friends are also extremely supportive of a potential relationship between himself and McAdams, and think breaking things off with Mendes would be the best thing to do.

"None of Ryan's friends think Eva is the right woman for him, and now that Ryan has admitted he and Eva are having trouble, everyone in his circle is trying to steer him toward Rachel," the source said.

Tags: ryan gosling Eva Mendes rachel mcadams