Hi carfan,
I do not know pxe stuff well, but my main suspect:
1)
check bios settings on these machines

I feel, at boot time, pxe is first device ;) bios searchs and fails, than boot continues with
Win7PE SE
well if main suspect is not the case, my other suspects:
2)
instead of Grub4dos, try using IsoLinux
Win7PESE\Main Configuration\"Boot Manager" - IsoLinux
3)
instead of Grub4dos, try using normal boot
Win7PESE\Main Configuration\"Boot Manager" - Auto
==> but you will loose otheros plugins, memtest ;)
3b)
you can also tset by changing used grub4dos version
Win7PESE\Finals\"4 - Create ISO" --> change attachment ;)
