I've tryied bitcoin-qt 8.2 RC1 as you suggested. Thing seems improved but remains pretty bad... Here is my updated situation. I'm returning again to bitminter... With P2Pool I've wasted much GPU power...
Moreover with bitcoin-qt 8.2 RC1 I see a message everywhere that's scarying me a little: Warning: (from bitcoind) Questa versione è una compilazione pre-rilascio - usala a tuo rischio - non utilizzarla per la generazione o per applicazioni di commercio In english it means "This version is a pre-release - use it at your own risk - don't use it for generation or for commerce applications" Should I worry?