Hi @Davie,
It may be an issue with low specs, but may also helped or solved by downloading the latest version of QGroundControl which includes a fix for this kind of issue as per this post. We don’t fully support QGC 4.1 at this stage, but the only remaining known issues with it are aesthetic, and as mentioned it can help with video issues. We’re currently in the process of testing 4.1 more fully and will hopefully switch over to recommending it to everyone soon, along with relevant guides/documentation to ease that switch 
I’d suggest you try it out and see how you go. The interface is a bit different, which can take some getting used to, but is generally easier to use and less in the way. Feel free to post if you can’t find a feature or have trouble getting it set up correctly.
Reiterating @murf’s suggestion of an i5 or higher CPU if you do choose to upgrade computers. The QGroundControl system requirements give a good idea of the minimum recommended specs for using QGC, so for a system that’s also potentially running other programs at the same time you’ll want those or a bit better if possible.