Quoting: Stupendous ManSo, is my card broken, or are the nvidia drivers acting up? The drivers are version 396.54 and I'm running a 4.18.12 gentoo kernel. I've been running the same driver for weeks before the errors started.

Same driver, but not the same kernel?

Quoting: tuubiSame driver, but not the same kernel?

I upgraded the kernel a week or two before the problems started showing. But you are right, I should try downgrading and checking. Not sure if the kernel I used is still available though.

I just tried downloading to a much earlier kernel, but no luck; I still get errors.

According to this documentation one of those error messages (number 69) could be HW related. Maybe try reinstalling or a different driver? If that doesn't help, it probably is your GPU.

That's a great document, thanks! Apparently I need to practice my Google-Fu as I did not find it before.

I'll investigate and see if I can solve this.

Quoting: GuestWhat happens if you run the Unigine Heaven benchmark for a while.

That runs just fine, I've run it 3 times back to back and no errors.
Yesterday I re-seated the RAM sticks and recompiled the Nvidia drivers, at the moment that seems to have helped. I've been running Eurotruck Sim 2 for an hour and again, no errors.
I won't call this solved just yet, but it looks promising.

EDIT: I'm still getting errors, and they all have one thing in common: defective hardware. I guess I'll have to get a new GPU. Can anyone recommend a good AMD GPU in the 200-300€ price range? I'm not going with nVidia anymore, I'm tired of the drivers not being open-source.

Cryptocurrency mining went bust. So, there are plenty of used cards on ebay right now.
Get whatever you can get that will give you good performance with Deus Ex: Mankind
Divided. The vegas are too expensive for your price point. The polaris line of cards
you can find good prices on. Try an 8G Radeon RX 580.

I am very pleased with my RX 560. In the price range you want the obvious choice is the RX 580.

In the end my trouble appears to be a faulty RAM stick, not the GPU, even though all errors have been graphics related. Memtest hasn't even found any errors after 9 hours of running, but if I remove the faulty RAM the system is completely stable, put it back in and games crash after 3 minutes. Weird.

Thanks everyone for your suggestions anyway!

Looks like you ain't parting with that GTX 660 for a little longer. 9 hours is quite the test, I once had a similar issue and the errors only showed up after 2 hours. It's good your nightmare is finally over ;)