iPXE undionly.kpxe Emulex 10GB random hang

[webminster]

Am experimenting with using undionly.kpxe (built from current sources) on various machines, and have an issue with an HP system with an Emulex 10GB NIC (xxx). Some of the time, it all works fine. Some of the time, however, the boot hangs at the iPXE "Press Ctrl-B for the iPXE command line" prompt, and never continues until I reset and try another cycle. Actually pressing Ctrl-B does nothing, no command line.

The card is a "07.00.0 Emulex Corporation OneConnect 10Gb NIC (be3) (rev 01)", "Emulex 10Gb UNDI, PXE-2.0 BIOS v4.0.493.0".

I can tell from Web server logs that in these cases, it never fetches the iPXE script, apparently hangs before that. Do see it getting an appropriate DHCP response.

Are there known issues with Emulex in current versions? Are there hints to debug these sorts of behaviors? Thanks in advance, please let me know if I can provide more detail.
-Alan