Shareaza and Gnucleus are both great for debugging your client.
The System window of Shareaza shows you all task it is performing (Send Handshake, etc).
In Gnucleus you can go to the Connections tab after you have connected your client with it. Then you can right click on your connected client and see some stats like the effiency and errors your client caused.

I used to do that way.

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