5 GHz Ping spikes - RT-AX88U

[mobileineurope]

My RT-AX88U on Merlin 384.16 (dirty upgrade from .15 - which was a clean install) gives us ping spikes up to a second - sometimes every 2-3 minutes. My son is a gamer - and is frustrated He is the only client on the 5 GHz net (channel bandwidth 80 MHz) - everything else is on 2.4 GHz.
It happens if we share some bandwidth with neighbours. If I select a clean DFS channel - we do not see the problem. If we could just stay there all would be fine, but the DFS channel detection sometimes (every 2-3 days) detects radar and switches to non-DFS.
What I am puzzled about is that the interference from the neighbours should be minimal. We live in a residential area with good distance to our neighbours. The strongest other 5 GHz net seen is at - 84 dBm, should this really be causing so large ping spikes every 2-3 minutes?
If I use another (non-DFS) channel with (low) interference it is the same pattern. Only a clean channel makes the problem go away.
Any suggestions?

 

[L&LD]

Can you not select a Control Channel that doesn't get affected by the neighbor's settings?

What channels are available to you? Can you post a screenshot?

 

[mobileineurope]

This is how it looks when DFS is OK. I tried several combinations in the non-DFS area but we still get the ping spikes.
I find it strange that the networks shown should cause so much trouble. When instead my son goes to the 2.4 GHz network - there are no significant ping spikes. And the 2.4 GHz channels are much more congested. Off course we don’t get the speed as we do on the 5 GHz.