63 is still way too hot for a CPU at idle. We have a all in one pc at work that idles at 65 degrees celsius and has the same exact issue you have with the NVR; unexplained random reboots with nothing in the logs to indicate why.
I would take the cover off & replace the stock fan that it came with with a better fan. Depending on if the noise bothers you you can choose higher cooling capacity at increased noise levels. But you are not limited to the same size fan either that it came with. With some creative mounting methods, you can install a bigger fan that moves more air at similar power draws but at quieter noise levels.
If you want to keep the same physical dimensions as the existing fan, get a replacement that is the same size but has higher wattage draw than the one currently in the nvr. taking the fan off the heat sink will show the current draw in watts or amps on the label on the back of the fan.
To be safe, do not install a fan that uses more than probably 1.5x the wattage of the existing fan, possibly up to a 10 watts max, or 1amp. Just depends on the RPM the fan produces. Some small high rpm screamers will do quite well with ratings of 1amp or less but produce much better cooling than the existing fan.
one other thing you can tryeven before the fan replacement, is just take off the existing heatsink/fanassembly, if it happens to be held on with screws/bolts/nuts & reapply new thermal paste to the cpu & clean the old stuff off the heatsink & then remount it.
Might be all it takes.Thermal grizzly makes great thermal paste that can drop temps in cases like yours.Who knows, maybe someone at the factory had a bad day & they didnt put any thermal compound on it at all, or maybe a tiny dab & then it passed through QC because the thermal shutdown limit was never reached during testing., though after extended use times, heat soak will overwhelm the capacity of the cooling system it came with & so it was never caught at the factory.
Your problem is, thankfully, almost guaranteed to be likely solvable so just continue forward with the hand you were dealt. Good luck
