2019 27" imac

So I have a 2019 iMac Retina 5l 3 GHz 6-Core Intel I5. I love it. I'm a musician and do pretty heavy duty production via Logic and the iMac has handled it with aplomb. So far. However, I've decided to delve into the video side of things, for starters recording myself while I play guitar etc...

I purchased a 2019 27" iMac with a Pro Vega 48 8g graphics card and 128g of RAM (3rd party that I installed). I bought it in May of this year and started having issues with it in October. I would be editing in Premiere Pro and it would just randomly shut down. It only shuts down while editing...