Video is lagging/choppy on FCP playback but audio is perfect

Hello,

So I've been using FCP for 2 weeks or so and I have been editing videos without a problem but today something super weird happened. After editing everything to how I like, I played the video back and it was suuuuuper laggy/choppy but the audio was perfect (in quality and timing). When I pause the video for a few seconds though the video will sync back but then go back to being laggy after another few seconds. I tried exporting it in hopes that maybe once it's exported it'll work but, no luck. The whole video wouldn't even export. The video is about 4 minutes long and only 1:30 would export. I'm really confused and frustrated and hope someone can help!

Also, I imported the original footage to an external hard drive so my computer would have more space. Not sure if that makes a difference but I figured I should add that just incase.

Thank you so much!

Cristina

MacBook Pro OS X El Capitan Version 10.11.3

Processor: 2.4 GHz Intel Core 2 Duo

Memory: 4 GB 1067 MHz DDR3

Final Cut Pro: 10.2.2