Yeah, same PC works fine running Linux from a bootable USB, so it's not a hardware issue.
Ha, you read my mind mate; just realised I had an old PCI WiFi NIC lying around (it's not anything great, one of those cheapo things off Aliexpress) and gave it a try with the Ethernet disabled. Seems the NIC isn't at fault then - timeouts and connection reset errors are also occurring through that card.
I'm wondering if some VPN or packet monitoring software is interfering with it — I thought I disabled them all when trying to fix this the other week, but maybe not. I'll try uninstalling Wireshark and my VPN apps to see if that restores any stability.
SMARTY (Three) 4G — 50 Mb/s down - 25 Mb/s up
TalkTalk Fibre — 14 Mb/s down - 0.8 Mb/s up
Edited by thebigdolphin1 (Sat 14-Aug-21 23:50:13)