I may be the only one who has had this problem. I couldn't find another post regarding this so I thought this might be help to someone else.The problem:
I have found that getting the network shield to turn on successfully every single time was essentially not possible with the off-the shelf hardware.
I have been using the device with 5v into the USB port and also 7.5v into the power socket so this isn't a voltage issue (at least I don't think so).
With either 5v via USB or 7.5v as described above the network board would turn on 90% of the time.The solution:
I've been using the ethernet shield PCB from Adafruit.
After careful examination I discovered there is a reset pin on wiznet chip that is not connected to anything on this PCB (other that a via that is).
I have simply run a wire from pin 8 (you could use any free one) to the RST via on this board. In my startup code i do the following:
This resets the wiznet chip itself and now I get a network connection every single time i power the device on.
Hope this helps someone.