Sources with direct knowledge tell TMZ ... a resurfaced viral video that appears to show JT looking what some have suggested is "under the influence" or out of sorts is simply being over-analyzed way too much by folks looking to unfairly condemn the guy.

While online trolls have said Justin looks out of sorts in this clip -- which was filmed and posted in May during one of his Vegas concerts, and which has since gone viral in the wake of his DWI arrest -- our sources tell us he wasn't even close to that state of mind.

Like we said ... some have openly wondered and/or speculated that Justin's wide-eyed gaze indicated he wasn't sober -- but we're told he 100% was, as he has been for every show.

Our sources say JT is always focused and clear-headed at all his shows -- including all the ones he's performed at while on tour lately -- and the one from May is no exception ... despite what people *think* they see in this footage.

We're told that, as a performer, you'd have to be sober doing what he does ... there's simply too much at stake in the performance.

As you know, all eyes have been on JT after his arrest last week ... where he was pulled over in Sag Harbor, NY for allegedly blowing past a stop sign and swerving on the road.

The *NSYNC alum addressed the legal drama at his Chicago show this past weekend, telling the crowd ... "It’s been a tough week, but you’re here and I’m here, and nothing can change this moment right now."

His lawyer has indicated they plan to fight the charge, however ... with the attorney telling TMZ he plans to "vigorously" defend the singer, who has a court date on July 26.