When you put any load on a graphics card the temperature usually rises pretty smartly but, 75C is fine but, it could be that the load is too much for your PSU. you say it 7 years old so, perhaps it is simply tired and beginning to fail. I would suggest you replace it soon, to avoid it causing other damage if it fails completely.
Buy a new PSU online, then if it makes no difference, suggesting the PSU is not the problem, you can take advantage of the distance selling regulations and send it back. Just be careful not mark it when you install it.