I would be tempted to check through the bios settings on both working and non working machines, and determine if there are any notable differences.
If you are loading a mouse driver in your PCDOS image, try remarking it out (I recall previous threads where mouse drivers were found to cause problems).
There are also limitations on the size of hard disk that "regular" PCDOS can support, based on the limit of 32 bit LBA addressing, which equates to around 132Gb if I recall correctly. This limit also afflicted the original release of XP.
Finally, another user reported that PCDOS appeared to have problems imaging past around 26Gb.
If you have the option to go with WinPE as your PXE boot environment, now might be a good time to see if this gets around the problems you are experiencing.