We have the exsact same problem, and we are also a school enviroment with 6000+ clients
pxe e32 tftp open timeout.
can PXE boot the same machine 10 times but in 20% i get TFTP timeout.
Ive dug in the network tracing and fount out the LANDESK PXE server is responing late with the DHCP offer then the error occurs i have Tracelogs from Wireshark i can send. I think it is the PxeMtftp.exe application that are to slow at responding 20% of the time.
Out server is running WS2008R2 SP1 8GB ram 2Vcpu ( vmware) no load issues