I have have weird problems with Arduino Uno. I have got some times errors with COM-port, computer have lost the COM-port, maybe because Arduino have jammed? Then I have had problems with LCD-shield and ethernet shield, if both have been connected same time characters have not been bright on the screen. And some problems I have had with Ethernet Shield also.
But now when I use external power (12V 1,5A) I have not had same problems than before. So FYI, if you have weird problems with Arduino and any shields, first try them with External Power Adapter :)
ps. I'm not sure how much Arduino and extra stuffs need current but I think 5-12V and 500-1000mA power adapted should be enough. pss. This is my first post... :) psss. This is not going to be my last post...