You’re blowing it up significantly beyond it’s normal size, so that would likely be a part of it. There’s also a good chance that clip from the movie is also originally interlaced, which compounded when blown up by a factor of 3 would cause problems. And quite likely 29.97fps. If this is correct, it’s probably a “it is what it is” thing. But someone else might have some options for you.
How did you obtain that clip from It’s a Wonderful Life? And what’s the specs on that clip with regards to frame rate and interlaced/progressive?