@lperoma So I can make the generalization statement that time fixes your issue?
What I want you to test is this.
PXE boot into the error and then press s to get to the ipxe console.
Wait 20 seconds.
Now key in the following at the console prompt
chain tftp://192.168.3.253/default.ipxe
Does this work and continue into the FOG iPXE menu. In that the only thing different between not working and working is time? (I’m driving to a point here).
If time fixes the issue, lets place a dumb switch (unmanaged cheap-o like monoprice) switch between the building network switch and the target computer. Now pxe boot the computer does it boot correctly? If yes then its possibly a spanning tree issue with your building network switch. Make sure your building switch has fastSTP, portfast or whatever your switch mfg calls the fast spanning tree protocol. I might be off base here but it sounds like it might be spanning tree. I know that processor is pretty old, but I don’t think its cpu related.