Based on the detailed issue you've described in your post, it sounds like you've been through quite a bit of troubleshooting already! Upgrading from an NVIDIA 3070 with a 650w PSU to a 3080ti with an EVGA 850w PSU, only to encounter black screen crashes during dual monitor use, is certainly frustrating. Your efforts in trying to resolve this, including a full Windows reinstall, driver updates with DDU, RAM replacement, and even diving into dump files, are commendable.
Given the complexities of your situation, here are a few suggestions that might help:
1. **Power Supply Considerations**: It seems you've identified potential issues related to power spikes with the 3080ti, which can indeed be a concern with high-end GPUs. Although your 850w PSU should technically suffice, the quality of power delivery can vary. You mentioned not having a larger PSU to test, but if there's any way to borrow one, it might be worth validating your theory about power spikes causing the crashes.
2. **GPU Stress Testing**: If you haven't already, consider running GPU stress tests (like FurMark or 3DMark) to see if the crashes can be replicated under controlled conditions. This might give more insight into whether the issue is load-related or possibly a fault with the GPU itself.
3. **Downclocking the GPU**: As a temporary measure, you could try downclocking your GPU slightly using software like MSI Afterburner. This can sometimes stabilize a card that's sensitive to power fluctuations or has marginal stability at its default clocks.
4. **Check for BIOS Updates**: Ensure your motherboard's BIOS is up to date. Sometimes, compatibility improvements for newer hardware are included in BIOS updates.
5. **Investigate VBIOS Updates for the GPU**: Rarely, GPU manufacturers release VBIOS updates that can address stability issues. It's worth checking if there's one available for your specific card model.
6. **Re-evaluate Dual Monitor Setup**: Since the issue seems tied to using dual monitors, experimenting with different cables (e.g., swapping HDMI for DisplayPort or vice versa) or connecting monitors to different GPU ports might yield some clues.
And now, for a bit of levity amidst the troubleshooting blues: Why don't GPUs ever get lost? Because they always have their drivers!
