Dell Precision PXE boot issues

I was wondering if I could see if anyone has any insight into the issues I am having. We recently received about 90 Dell Precision 3680 desktops. We are having difficulty imaging them though because I have having PXE boot issues I am not exactly sure how to troubleshoot.

The issue is that when PXE booting, it takes 6 minutes for the computers to download the NBP file. Then once it finally downloads you get the prompt to press enter on the screen, but it only lasts for a few seconds and if you miss it, you have to start all over again. Once you get past the NBP file download and pressing enter, everything proceeds smoothly. I tried messing with the TFTP file settings in the server registry, but that didn't make a difference.

Does anyone have any ideas? We have a large number of Optiplex and Latitude devices as well as a decent number of Thinkpads and none of them have this issue.  I tried some of my older Optiplexs (3070, 3000) and they do not show this issue.  They get the enter prompt almost immediately.