If you have MiamiDX then use the mni-drivers coming with miami.
Set the cards to halfduplex. (You can change that later to fullduplex)
I had to delete everything on the PC concerning VPN.
MY network (curently)
PC 3COM Combo pci 192.168.0.2
A4000T ariadne 192.168.0.3
A2000 ariadne 192.168.0.4
Try to connect the amiga via a crossover-cable directly to the pc.