unstable lan with EP-808AX over 2 connections

I have been successfully my surecom EP-808AX switch with the following
configuration:
- 1 adsl router modem (routing of 4 IP addresses) (zyxel p650 me)
- 2 pcs (1 laptop, 1 desktop) with windows XP professional
- 1 xbox

now (since a few days) my lan is:
- working fine with 2 connections (whatever device combination I use,
so it's not xbox or modem related)
- rarely working with 3 connections, and the time before all 3
connections are up is longer (a few seconds at least)
- apparently not anymore with 4 connections (or perhaps I do not wait
long enough)

by my lan is "not working", all the connection leds are extinct,
except for some quick flashes where each cable is connected, about
every second

--> I've read about some power supply issues. I may have to check
that.

--> I've read about the MAC address learning capability, with a 5 min.
timeout, so I tried to wait that time, but it didn't help

If anyone has a clue about any other probable cause, please tell !