The stalker problem in Hollywood has gotten so bad, that celebs are now having to hire PIs to track down these people to make sure they stay away … including a famous violinist.

We did a ride-along with a seasoned vet in this industry — a private investigator named Joe DeLu … who was recently contracted by Lindsey Stirling to locate a man she claims has been stalking her and making her life a living hell.

Check out how DeLu explains this phenomenon to us in the car — he says there are a lot of law enforcement agencies these days that don’t want to serve stalkers with stay-away orders, out of fear of being attacked and harmed … and as a result, there’s no one left to do it.

That’s where Joe comes in … and you can see him doing the job here — confronting a man who’s alleged to be obsessed with Lindsey.

BTW, this guy has ties to hounding Taylor Swift too — he’s been obsessed with her for more than a year — which just speaks to the larger issue.

Anyway, the entire interaction here is part of a show airing Monday on FOX called Obsessed and Dangerous: Hollywood’s Stalker Crisis’ — kicking off a new series that’ll start to air weekly. The show starts at 9/8c … and you won’t wanna miss this.