Yet another issue - M60

I only experience this minor issue when the car is cold. I crank and it idles fine but when I put it in gear and start to move it starts stumbling but not to the point where it's gonna stall, just about to 500rpm (idle afaik is 650) it only happens when I first move it, doesn't happen after that until I have another cold start (sorry if it doesn't make that much sense)

I think I have a vacuum leak coming from one of the intake gaskets because they're old and have never been changed. What else could it be?