@Marzipan Mine has been cutting out as well. Couple times a week I was having to go to the basement and power cycle the modem. It is in the server rack but it relies on purely passive cooling. So even though the rack doesn't get above 28'c internally I think it was over heating.
Leaving the doors cracked slightly past couple weeks and it stopped happening so I zip ties a fan to it last night and will see if that works. If this works long term I might revisit this with a better sized fan and a printed bracket to hold it.
If these can't even be in a 28'c ambient space then some are probably prone to intermittent failure just being in the wrong spot, near a window, below a heat vent etc.