Our Firestick used to stop about seventy minutes into anything, reliably, and I replaced it once before working out that the problem was not the stick.
It was where the stick was. These plug into the back of the television, the hottest part, in a gap with no air, and then run a video decoder for two hours. It gets hot, throttles itself, and what you see is a picture that degrades and then stops — at roughly the same point every time, which is the clue I ignored for months.
The seventy minutes was the tell. A network problem is random. A heat problem has a schedule.
The fix cost nothing because the extender cable was in the box, which most people throw away, including me. It moves the device a few inches into moving air, and that was the whole solution.
Two related things. Wall-mounted televisions are worse for this because the gap behind is smaller. And powering the stick from the television's USB port rather than the mains adapter causes a similar-looking but different problem — under-powering, which produces stutter and random restarts. Use the adapter it came with.
Which device to choose, and how to set it up so it does not throttle, is set out here.
Seventy minutes, every time, and the answer was in the box.