Marc,
Do you happen to have the product (switches)that you are using so we can assit you?
1. Is there an issue with getting ip addresses when booting up?
2. When the PXE is complete, are the clients expecting to get a new ip address?
The questions above would help as well as the firmware that was running at the time all was well.
Was there an upgrade since that time?
Also did something change in the configuration (You can compare images if you do back ups with NetSight ).
Jason