Why does the router cut the wired Internet speed?

Question

network hardware

Good evening.
Faced such a problem, the Netgear jwnr2000v2 router worked for three years without any complaints at all. But recently I began to hard cut the speed of the Internet connected via a wire - instead of 75 Mbps, it gives 5-25 Mbps. I updated the firmware - it didn’t get better, turned off Qos - the same problem. Although earlier through the wire it stably kept the maximum speed of 75 Mbps. Tell me at least where to dig, otherwise I've been fighting for 2 days. I checked the speed with speedtest. When connecting the wire directly, the speed is declared 75 Mbps. domra provider.

Answer 1

monitor its temperature
, maybe it overheats, cuts the speed of the processor, which then leads to a decrease in the speed of the Internet