Everyday story of simple folk:

Took the side off the case. The temperature of the GPU rose quickly to 97C, 15 seconds or so, so turned the computer off fearing it was going to fry the card.

Purchased, at unbelievable expense, tin of blow dust away stuff: only reduced temperature by a couple of degrees.

GPU temperature: 81C
Fanspeed: 100.0%
nvidia-settings has just issued a dire warning about voiding warranty on whole computer if change clocking of GPU.