@Redbob This reminds me of a communication problem. You should not get a connection reset…
It kind of makes me think its a spanning tree issue or a faulty cable. But if everything is the same other than bios/uefi switch on the same computer it really can’t be communications. If it is a communication issue, can you confirm the switch that the pxe booting client is on is using one of the fast spanning tree protocol like Fast-STP, RSTP, MSTP, port fast, etc? Another test would be to put a dumb unmanaged switch between the pxe booting computer and the building switch as a test. If it works with the dumb switch then its a spanning tree issue with the building switch.
If the firmware up to date on this target computer?
The other thing to test is to put the pxe booting computer on the same subnet, same switch as the FOG server. This would rule out any devices in between the two causing this issue. It still makes me think of a communication trouble because its failing at random different steps in the booting process.