I have seen this when PXE booting DOS (MS-DOS and FreeDOS). What I had to do, many disagree with this, is rebuild the DOS PXE configuration.
So launch PXE Configuration Utility | at the boot menu in the lower right hand it says "Regenerate Images". Select DOS and click the regenerate button. Once the process comples click Save to commit the changes. Wait for the progress bar to complete and click ok to close the Utility. PXE boot a client select your DOS pXE image and see if you see the same issue.
nelo