Run it is a Windows service. It is in Blue Iris settings under startup.
The shutdown can be either because you have a memory leak with the graphics driver. Look for that in the wiki.
Or sometimes it just happens. Mine will run fine for weeks, then do a restart for a couple of days, and then run fine for weeks.
The restart will assure you do not miss a thing so make sure you check the run as windows service box. Literally I was on it watching and it restarted and didn't miss a beat. It restarts fast.