This is a wild guess, but your hard drive might be the reason if you got an hybrid hard drive (it probably came with the machine)
Hybrid hard drive users experienced random freezes from time to time, specially old hybrid drives. It could be a firmware issue, or the solid cache of the drive might be wearing off. You could try to upgrade the firmware or to disable the nand flash completely to see if it fixes it.
Also I think your notebook has 2 hdd slots, if this is the issue, adding a second one or an ssd and booting the os from there it will solve it as well.
I can't think of another issue that might be causing "strokes", but that doesn't guarantee there isn't one. The tough part is that you will not know for sure for a long time if the issue was fixed since it only happens randomly 😛
You should google: seagate hybrid freezing
And read the experiences of other users to see if your strokes are similar to their freezing. Hopefully you will find the root of your problem. Finding out your exact model of your hard drive and google the issues it may have might be helpful as well...
When I experienced freezes I just waited a bit and added an ssd to the machine and I just used the seagate drive for media and temp file storage. I never upgraded its firmware or disabled the nand flash. If you are going to do this make backups or research if there are any risk associated with it.