lspci -vvv -s 04:00.0
She pushed the patch to the netdev mailing list. The subject line: [PATCH net] r8169: add missing wmb() before re-enabling interrupts on RTL8168 VER_52 .
cd /usr/src/linux/drivers/net/ethernet/realtek/
That was the gap. A single misplaced memory barrier. A missing wmb() (write memory barrier) between writing the "I'm ready" flag and re-enabling interrupts.
The first ten seconds: perfect. Twenty seconds: perfect. Sixty seconds: zero drops.