Thank you for your detailed explanation.

My 2865 is in a business environment - has 3 devices plugged into the LAN ports
1 - gigabit POE network switch
2 - another router - Cisco
3 - gigabit POE injector for the point-to-point aerial - Mimosa

I experienced two major issues so far
- when using firmware 4.4.5.3 & 4.4.5.4 - 3rd port where the POE injector is plugged in - kept on disabling itself completely - small LED lights on the port were off
- when using firmware 4.4.5.8 - the whole router was restarting itself every 15-20 minutes.

As far as I know, I can't change flow control on that POE injector or the aerial which is plugged in through it.

Thanks Geroi .
Until competitively recently I only ever used port 1 connected to Cisco switch, but it's only due to a change in physical location of the router required in preparation for full fibre via an ONT, that I use the other physical interfaces on the router for convenience. This however was the situation for several months prior to the recent flurry of firmware updates past 4.4.5, and prior to me changing my Internet connection to fibre via the WAN interface. It had previously been very stable other than the loss of data throughput after about 5 weeks - easily solved by a scheduled reboot at 3am on the 1st of each month.
I'm sorry to say that the hardware flow control is a red-herring, or at least partially - I now takes about 4-5 hours before the router reboots with that PC turned on and connected to port 5.
I have now moved that PC to port 2 - a port I have previously plugged in my backup "server" of absolutely identical hardware (bar the hardware flow control config - now rectified) and ran for many days without issue on 4.4.5.8. We shall see what happens, but it is looking like for me at least, it's using port 5 that is the issue with 4.4.5.8. If all else fails I shall have to add a network switch in the same room instead of relying on the one built into the router. How naff is that!
For what it's worth, I do have hardware acceleration enabled in the router else I don't get the full benefit of my full-fibre connection speed.
I should know by tomorrow if the change of port connection has achieved anything for me.