>I would tend to blame the 3c509 for this. I've got tons of problem with them,
>because of their really small fifo. Could you try another ethernet
>board (ne2000 are good, but a DMA-capable board would be better for
>a server).

Um, real ne2000s or ISA clones are _not_ good.
The realtek 8029 or other clone is better, 


>In a general manner, I now tries to avoid 3com networks adapters (althouh
>their other networks products seems good).

The 3com 10/100 line okay, the fifos are adequate for 50 mbit/sec or
thereabouts.  I've found they can actually be a win over dma on some
of the less-well-performing pentium-pro PCI chipsets.  The only
problem is the 3c905b, which dropped compatibility with the 3c509
programmed I/O model and is dma-only.  the driver in -current doesnt
support it yet, though.