I fixed it. I tried a lot of things - reinstalling network drivers, reinstalling video driver, repairing the game, rolling back a recent BIOS update, changing the priority of eso64.exe, etc. None of that worked. Then I had a thought - why only during combat within trials, what's different? Logs. I turned off logging and deleted the file that was there - problem solved. After running for a while and having no issue I turned logging back on to see if it came back but it was fine so the issue must have been with that specific log file - maybe it got corrupted? Anyway, there you go.