Why is movie running slow every 30 seconds or so?

[Bubblegum]

Hello,
Sometimes when I watch movies on my laptop the movie slows down every 30 seconds or so. The movie runs fine, but at every 30 seconds(or so) it slows down a bit and then continues normally. It looks like it's a scene in slow motion or something when that happens.
It doesn't always happen: usually when I watch something short, like 20-30 mins, this doesn't occur.

Do you have any ideas why and what I could do about it?