What do you mean by "tried changing file types" ?
Watched a movie earlier today had no issue. Wife started another movie about 20 minutes ago, and so far no hiccups (I would have surely heard about it) :).
Sure it's not your network ? Hardwired or WiFi ? Tried rebooting your router ?
ISP throttling maybe ? Did you try enabling a VPN, just in case ?
Would probably help to specify what movie you are referring to. Maybe it is the specific movie that has an issue.