Hello Ahmet,
I will try to give you as much information as you need. So let's see your list first:
1) I tried different drivers. I tried them with both versions 3.08.5 and 4.0 RC7. Not every single driver, but at least 4 (388,398,411,416) it didn't seem to matter. If you think it might be useful to go through every single one of them, I'll do it. But how do you mean to have a clean install? Do I only uninstall the old one and restart or do I have to use some sort of cleaning program?
2) no overclocking
3) PSU should be good enough. It's a Seasonic X-Series 1250W, 80+ gold. I have 3 GPUs.
4) As I mentioned it before GPU's VRAM is not full. It lists as I still have 2-3Gb available when it crashes. I also enabled Out of Core. (how high should I go with that? What about GPU head room?)
5) Crashes are usually come with CUDA errors, but there are some random Octane crashes.
What information you need exactly to investigate this issue? I will try to reproduce the errors and give you a file with it. In the meantime, this is the usual error message (attached).
I saw you mentioned something about not using the monitor GPU for rendering. I will give that a go, even though I'm not too happy about not being able to use one of the graphic cards, that will slow things down.
Thanks a lot, I appreciate the help and all the hard work you are doing. This software is awesome! Keep up the good work!
PS. I will also try NOT using out of core, as screamerclauz mentioned.