@pamadmax Its great that you have it solved. Intermittent communications would also cause this pattern.
Just to clarify the point a bit about dual dhcp servers. If your fog server is on the same imaging network and its “the dhcp server” for your imaging network, then you don’t need a dhcp-relay service listening on this subnet. Actually you should clearly not have it listening on this subnet. You can have the dhcp-relay service on your router, that’s fine, but not listening on the imaging network’s vlan.
The issue is this, if the dhcp-relay service IS listening on the imaging subnet, the relay service will relay the dhcp request to your main server. With the FOG dhcp server also listening there is a chance that your pxe booting computer will get two OFFERS. One from your FOG server and one from the dhcp-relay service as a proxy for your primary dhcp server, hence the comment about 2 dhcp servers.
You have it worked out, so that is all that really matter here. Good going on finding the problem. I’m sure that misconfiguration on the LAG trunk was causing other strange issues on your network too.