Hi kreciu,
I have got fastethernet.device working completely stable for a few years here. Is it Realtek 8139 compatible card? I have a Realtek 8139D, Voodoo3 3000, and a SB128 card all working together. If it seems like a hardware fault, you could try re-ordering the cards or run c:pciinfo to make sure no two cards have the same IRQ. Check elbox.com to make sure you have the latest fastethernet.device. I use my old Miami TCP stack, but it also worked in the RoadShow demo as well.
If you have any other questions I'm happy to pop upstairs and crack the 1200T open and have a look for ya!