I have seen this before on PlayStation and Xbox but the cause is usually the same for a PC. The system is struggling to render graphics, while it runs background code. Anything that runs 3D graphics can suffer from this, not just games. Older system are hit hard. I have a weak system and play on the lowest settings possible for SS2.
Your title is Older PC build. How old are we talking? If you can answer some of my questions, I might be able to help. What hardware does your PC have? RAM, HD or SSD, video card, ect.
Anti-virus software can also play a factor. Overall, it could be many things, like your computer keeps trying to update while you are playing and you don’t know it is doing this.