imgfetch stuck mid-download

[Geth]

Hi,

I'm a netboot rookie and I'm using iPXE version "aa4b0", in BIOS mode generally ipxe.pxe (more on that later).
First, I've wanted two things to work: Memtest86 and WinPE(10) with EFI and BIOS mode.
EFI mode is working OK, both stuff. (Memtest86 version is Memtest86 7.0.0 Beta 1 -- I cannot get it to use supplied mt86.cfg, but I think it's a Memtest86 bug).

Now the BIOS mode.
I want to use Memtest86 4.3.7 (NOT Memtest86+ 5.0.1).
If I just chainload it from iPXE, it fails with known error 460381.
Going the longer way (iPXE -> pxelinux 6.03 -> memtest) it's working:

set filename lpxelinux.0
set next-server 192.168.x.y # same server
set root-path http://<local http server address>/
chain ${root-path}${filename}

Done. Going for WinPE by the book (latest wimboot), so there are a couple of imgfetch calls.
Sometimes it works, but more often it gets stuck in the middle of a download (sometimes it happens in the middle of a font file, other times it's the .wim file). It's not frozen, it's just not advancing.
(I could try a timeout loop per file, but it's a working LAN environment, it should work hassle-free.)

I've switched back to undionly.kpxe. It boots 3 out of 3 times successfully, no hiccups.
Unfortunately, now the previously working memtest part fails:
Booting kernel failed: Invalid argument

The BIOS PC is equipped with a onboard rtl8168 (LAN ROM 2.59).

So, I would go with ipxe.pxe instead of undionly.kpxe, but than I don't know why imgfetch is not working ALL the time.

Any ideas? What other info would help?