@kas1e
I thought that RTL8139 only works well under Windows

The only person who did not report any problems was using Windows (he did not specify which driver version he was using)
It does not work properly on any emulated machine (A1, Peg2, and Sam460 - official driver even 53.11 and unofficial 53.7).
What version of the RTL8139 driver are you using?
The 53.7 driver can be easily identified even when starting the system with debug
it prints additional information that is not available in the final versions
--
[HW_BoardReset] PCI Vendor ID...: 0x10EC
PCI Device ID...: 0x8139
PCI Capabilities: 0x00000003
PCI IO Range....: 0x00001200
[HW_SpeedSetup] Unit 0: 100Mbps full-duplex operation!
[HW_BoardInit] Board at unit 0 ready for operation!
--
There was a discussion about problems with RTl8139 two years ago. You would have to go through all these topics, which could take a long time.
I also don't remember how MorphOS worked with RTL8139 and Qemu.
@joerg
I changed the IRQ for A1 and PEG2 - it didn't help. The problem was the same - complete disconnection of the driver/device after a while.
RTL8129 does not work under QEMU and ASO4.
The best test would be to insert an RTL8139 PCI card into a machine with QEMU PCI passthrough.
Pass the RTL8139 card directly to AOS4 on Peg2 or A1 emulation and check.
My machine is packed away in a cupboard. Maybe I'll run it when QEMU-11 comes out.
If anyone has a machine with QEMU PCI passthrough, they can check it themselves.