When you say sometimes it will sometimes it won't, what kind of error is it giving and where does it fail?
We had some new Lenovo systems that would sometimes PXE boot fine and other times would give us a "windows failed to start" and a "a required device isn't connected" error. Or other times we'd just get TFTP errors when it was trying to pull down the boot.wim.
The fix ended up being having to go to Self-Electing subnet Services, and under PXE Service, find the PXE Rep and turn TFTP Block Size down. I think the defaults are 16384 for ia32 and 65464 for x64. We ended up having to turn ours down to 8192 for ia32 and 32768 for x64. Apparently some systems just don't want to download the WIM file at faster speeds. Also, remember any settings changes you make here take 15 minutes (or whatever your Polling Frequency is set to).