In the end this issue was caused by my Virgin Media "SuperHub 3.0" router with it's dynamic wifi channel switching.
I disabled it about 11 days ago and not a blip of an issue since.
This also explains why it occurred more often when I was using my laptop upstairs in my house, something which I thought was just my imagination, but obviously an action that ...
There is NO Raspbian Linux 4.19.81-v7+
You probably have Raspbian GNU/Linux 10 (buster) running kernel 4.19.81-v7+
You have probably broken your system by running rpi-update
"In normal circumstances there is NEVER a need to run rpi-update as it always gets you to the leading edge firmware and kernel and because that may be a testing version it could ...
Here's the relevant part of the official docs.
Min and max:
GPU memory in megabytes, sets the memory split between the CPU and GPU; the CPU gets the remaining memory. The minimum value is 16; the technical maximum value 944 [for the RPi 4]. The default value is 64, values above 512 will not provide increased performance and should not be used.
You shouldn't need to set it beyond the default value in the plain Raspbian /boot/config.txt. Max value is 128M.
Setting it beyond 128M can leave your Raspberry Pi 4B unbootable.
GPU memory on a RPi4B isn't used in the same way as on earlier Raspberries.