I have recently found that the Optiplex 980 with Bios A09 and A012 experience a extremely delay when PXE booting.  The delay was anywhere between 1-2 hours but it would eventually boot the KBE Main Menu. 

I believe there are other Bios versions effected due to my research on it from other users but for sure what I tested was A09 and A012.  

The newest revision of the BIOS A13 release Feb. 1, 2013 fixes this issue.

Flash the bios to A13 and this will fix the issue.

http://www.dell.com/support/drivers/us/en/04/DriverDetails/Product/optiplex-980?driverId=MMTX7&osCode=W764&fileId=3114093175

Also this link is to flash a bios without having the OS on it.

http://ubuntuforums.org/showthread.php?t=1901977