Trouble for Nick & Vanessa?

Are Nick Lachey and Vanessa Minnillo on shaky ground?

The couple were spied sharing a decidedly “tense” dinner at the Cheesecake Factory in Sherman Oaks, Calif., on Sept. 25, sparking concern that the pair are on the verge of a breakup.

Although Vanessa’s rep insists, “They are still a couple,” an eyewitness tells OK!, “They appeared to be extremely tense and irritated with each other. They were having some sort of verbal tiff at their table, and they looked miffed.

“They looked dour when they sat down, annoyed over dinner and still upset by the time they left the restaurant at the end of their meal. It may have been a minor squabble, but they looked like they’d rather be anywhere but with each other.”

Nick and Vanessa have been dating since 2006 and living together since 2007, splitting their time between NYC and L.A. In the summer of 2007, the couple were wrapped up in a scandal involving racy pictures taken of them while on vacation in Mexico.