Failed with the 3090s… I need help choosing a GPU

Excerpt

I'm building a budget AI PC for our company's application. The specs are: MSI Z370 TOMAHAWK 64 GB RAM NZXT C1200 Gold PSU 2x RTX 3090 build was finished and I started testing with llama.cpp. After about an hour it froze, and nvidia-smi reported errors on the first card. I kept testing, checked the logs, saw driver errors, and tried various fixes — after a few reboots the PC wouldn't POST at all and the card was dead. OK, so I'm down to one GPU: more testing, and two days later the exact same pro