On 9 Jun 2015, at 13:16, Mark Rogers <mark@more-solutions.co.uk> wrote:

You say it's an outside cable... did you terminate it yourself?

Yes I did. I will reterminate, although it's not really a strength of
mine and I'm as likely to get it wrong this time as last time....

I rather like these: http://www.kenable.co.uk/product_info.php?products_id=3252 "Crimp ends are supplied with a crimp plug (body) and a separate guide plate, the Guide plate is removable from the main body of the crimp so allows you to align your cable outside the crimp plug and then slides inside the body to be crimped.". Easier to use, seems to work.

Stupidly, I had thought about swapping cables but ruled it out on the
basis that the length was too great, and had forgotten that the PC can
be moved....

:-)

It's a quad-core CPU with 24GB RAM,

Another thing to check might be the cooling -- possibly if the heatsink is not well connected and the cpu it gets too hot it throttles to a lower clock speed.

Check your powersavings settings too to make sure you're not running below what you think you should have.

But I'm sure you'd have spotted that by now.

It *feels* like something is interrupting the CPU and I've
pretty much ruled out all the local PC hardware by replacing it..

It might also be worth to try booting into a live CD or two see if they perform better, which would move suspicion to the OS.

Additionally: I've had issues with the Virgin router just locking up
and needing rebooting, or locking up on the cabled ethernet side but
being fine on wireless. When I put a cable router in between the
Virgin box and my home network, this issue seems to then start to
affect the cable router. That's what made me start looking at the
network initially.