[ENG] Strange router behavior causing extreme latency (GR241AG)

I have a FiberGateway GR241AG in my home. 

The core of the problem is that the router hangs a lot, causing extreme variable response times.
The problem varies in severity, but with no obvious pattern. 
We first noticed it when our cellphones where having issues maintaining WiFi connectivity. Since the wifi-antenna is missing, I though it was a simple solution, but the addition of an antenna did not improve the situation. 

I wanted to rule out someone else using our network, so I changed the password and SSID. At first it seems to have worked, but it was just coincidence. It returned to the same state a day or so later. 
Here I should note, changing the settings was a challenge as the HTTP configuration was extremely slow. Even after a full reset. 

Yesterday it was particularly bad, so i decided to ping DNS servers (1.1.1.1 & 8.8.8.8)  and saw very high latency with extreme variance: 4ms to 4000ms. 
Then i pinged the inside of the router (192.168.1.254) on a cabled connection, and the same thing:  <1ms to 4000ms. 

Here is a short pluck from the ping statistics from the 1.1.1.1 run, but the pattern is identical on the 192.168.1.254 run. Timeouts and all. 

Reply from 1.1.1.1: bytes=32 time=65ms TTL=59
Reply from 1.1.1.1: bytes=32 time=4ms TTL=59
Reply from 1.1.1.1: bytes=32 time=221ms TTL=59
Reply from 1.1.1.1: bytes=32 time=293ms TTL=59
Reply from 1.1.1.1: bytes=32 time=180ms TTL=59
Reply from 1.1.1.1: bytes=32 time=1281ms TTL=59
Reply from 1.1.1.1: bytes=32 time=142ms TTL=59
Request timed out.
Reply from 1.1.1.1: bytes=32 time=1853ms TTL=59
Request timed out.
Request timed out.
Reply from 1.1.1.1: bytes=32 time=468ms TTL=59
Reply from 1.1.1.1: bytes=32 time=151ms TTL=59
Reply from 1.1.1.1: bytes=32 time=18ms TTL=59
Reply from 1.1.1.1: bytes=32 time=1943ms TTL=59
Reply from 1.1.1.1: bytes=32 time=2721ms TTL=59
Reply from 1.1.1.1: bytes=32 time=3142ms TTL=59

To me it feels like the router is either compromised or dying. 

I wanted to try and disable either the 2.4 GHz or 5 GHz wifi-bands to see if there is a conflict there, but am not able to due to mesh topology and smart-wifi. 

What to check next?