It would be interesting for you to capture a Wireshark of the PXE process.
The following article illustrates it.
http://community.landesk.com/support/docs/DOC-6746
The client should ask for an IP address and for a boot server. It may be that the boot server information being handed back by DHCP is incorrect.
Check that the DHCP options do not include boot server information.
Also ensure that the PXE MTFTP service is running on your PXE representatives. Restart both PXE services for good measure.