Try what I posted earlier, also as electronics age they can behave that way, the 390 came out in 2015, so they are getting some age, they should still work for quite a while if properly cared for. Look at the card and see if there are any moved or missing components, carefully blow some compressed air to clean it of dust while holding the fan so it doesn't rev faster than it was meant to and damage the fan (yes you can damage a fan if you blow air on it and it goes too fast). Also make sure Windows isn't putting a driver on there, you can set windows to not update the driver, search the internet for how you can do that. Other than that you will have to isolate the problem, and it can be a real sleuth case for Sherlock Holmes sometimes. Good luck!
Also it is possible the thermal paste or pads have gotten old, you can replace them but I would want to be above a novice to try it because if you do not do the job right the card can be damaged beyond repair, so watch videos and I take no responsibility for damage if you do attempt it. If you do it, make sure the pads are the exactly the same thickness as the previous ones. There are also problems with the VRMs getting too hot or burning out on some cards. I hope your problem is only minor. Always check in your BIOS too to make sure you don't have a bad setting for PCIe. There are a lot of troubleshooting that you can search all over the internet. I hope you can get it fixed!
