Weak/no connection xDSL WinXP

Dear all,

Currently I'm having a weird problem, my PC cannot connect to the
router under certain circumstances.

I'm using:
- Windows XP Professional XP2
- DELL Dimension 4600 w/ integrated 10/100 Ethernet card
- Sitecom router DC-202v5 (firmware 1.97e4 (latest))
- xDSL modem integrated in wall socket (glass fibre)

I've tried the following ways to my PC connect to the router:
1- modem/wall socket - 30m UTP cat 5 cable - router - 1m UTP cat 5
cable - network card PC [works perfectly]
2- modem/wall socket - 30m UTP cat 5 cable - network card PC [doesn't
work: weak/ no connection possible]
3- modem/wall socket - 1m UTP cat 5 cable - router - 30m UTP cat 5
cable - network card PC [doesn't work: weak/ no connection possible]

Given the fact the wall socket cannot be relocated and additionally
that near this socket there's another PC to be connected (which works
in all cases above), what is the best solution? (Number 1 would not be
efficient, as it would require two long cables from room A to B and
vice versa.)

Thanks in advance,
theredspecial

addition:

- full/half-duplex settings make it slightly more confusing:
* automatic: weak/no connection
* 100 Mbps full and duplex: weak/no connection (both router and
ethernet card are 10/100 Mbps)
* 10 Mbps: half: Weak/no connection
* 10 Mbps: full: connection, IP assigned, but connecting to internet is
not possible