Collision Led is flashing with each packet

I am using an Uno with an ethernetshield, the combination is being used as a UDP streamer i.e., they just send UDP packets to my PC. The connection between the two is established with a CROSS-OVER cable.
Sometimes the two work just fine and sometimes when I power the board the "coll LED" will start blinking indicating a collision, the weird thing is that, this happens sometimes even with the UTP cable not plugged into the shield, so who is colliding with what?
When the problem happens it persists and the only way to overcome is to restart the unit. I googled such a problem and didn't find that much about it. So my question is: what does the "coll LED" indicates (what is colliding with what?), did any of u guys encounter such a problem? I know the situation is very vague but any clues or directions about how to think about the cause of the problem or how to debug it?

Thx in advance.

The only reliable fix I have found for this is pressing reset. Not a great option. I am increasingly of the view there are major issues with some of the w5100 shields on the market. Issues like not getting an IP address, collision led flashes, not being able to connect through a switch, dropped udp packets.