I have a defective pi.
In fact I have 2 pi's, one from Farnell and one from RS.
Let's call them the "good" and the "bad" pi

I can't say if the bad pi is from RS or Farnell... How to figure it out?
I can imagine sending a pi to the wrong company woudn't work.
What's not working:
The problem started, but after a bit of trying, it works again, now it isn't possible to solve it.
Ethernet leds are OK, DHCP requests are being sent, but aren't received by the pi.
It is even stranger that this works on a different router, but you need to be very patient between typing username and password when using ssh, so there is a problem, I'm thinking about damaged magnetics and that a gbit router has a stronger signal or something like that...
I have 3 SD cards, one in the good pi, one in the bad pi and one smaller SD card that's sometimes used in the bad pi to messing around.
All cards work in the good pi, and none of them works in the bad pi.
When I'm using a wireless dongle... it works always.