Port status LED turns off after a while

[rolfsson]

I have a simple 3Com Gigabit Switch 8. It works without problems when connecting it directly to my router with a short cable. But I run into problems when trying at longer distance (don't know if that's the problem though). From the router I have two cables running to other rooms, distance about 20-30 m, where they end up as wall mounted lan outlets. I have no problem connecting computers directly to these outlets and using the network. The problem occurs when I try to connect the switch to any of the outlets. In one room the port status LED doesn't turn on at all. In the other room it turns on at the beginning (green), flashes a bit, but turns off after 15-30 seconds. Switch unusable.

I am using the same cable for the switch in all three cases. Anyone with an idea of what the problem might be?