HELP! 66 cuda idles fine, but not when in gear

Hi everyone, been here for years, don’t post much...but I need some help. I have a 66 barracuda with a 408 stroker. I have a small Holley 600 carb on it. Motor has about 500 miles or so and she will fire right up, and idle great and stays running fine. It runs like a beast at wide open throttle, part throttle, you name it. The problem occurs whenever I have to slow down and stop at stops signs or lights. The car wants to stall as soon as I stop. If I put it in neutral, it’s fine, and then I put it in gear and she goes fine. It’s a very annoying problem that I can’t seem to solve, no matter what I set the idle at. Any solutions?? Please!