At First guess, I'd say you have a duplex issue. The empeg has to be at 10 mb, half duplex. The hub will only do this, and won't try to push it to full duplex.

The 3coms (and HP switches, as I have cursed before some firmware updates) autodetect what setting is needed. Sometimes they miss. They might get the 10/100 part, but the half/full slips by. And that can be fun to figure out sometimes.

Leaving the hub in the equation forces half/10mb.

The problem with standards is that there are so many to choose from.....