Sounded like too small of a debouncing cap/s on/around the nic, undersized vcc/gnd planes and/or too small of a power supply to handle peak current.

The other thing, once a system is booting, is always make sure RAM, storage, CPU and nics are all working, so test those individually: memtest86+ (or rowhammer or latest memtest86 non-plus), iozone, prime95, iperf or their equivalents.