So I was in cyrodiil and had been there for about 2 hours, I was in a fight with a sorcerer when suddenly the pc made a funny noise the screen went all pixelated and rebooted.

For sure it was the sorcerer! He casted the third Morph of Negate - Negate to play!

For me it sounds also like the GPU was malfunctioning. It isnt necessary that the GPU itself is the cause for your problem, it could also be the power supply, cpu or motherboard which made your gpu malfuntion, but the probabiltiy that it is the gpu itself is very high.

Usually something like this happens when the GPU overheats. Check your temps while playing (for example with HWiNFO64).

Since the game uses the multicore rendering it can use more of your GPUs power (thats why you probably have more FPS than before) leading into higher power consumption and finally heat.

Also there are some parts that might decrease over time leading into lower tolerance of high temperatures. The older your GPU is / the worse the manufactoring quality was the lower the temperatures get at which it can operate stable, leading to a point where it might crash when it didnt before.

In short: Check your Temps and / or try to disable multi thread rendering. Also updateing your gpu drivers is always a good idea even probably not the cause of your problem.