A known fault with the 5 & 6 series Onkyo's & Onkyo have done out of warranty board repairs for free (except for postage) as a goodwill gesture for quite a few people, Maybe this should be your first port of call.
If they are not forthcoming with a free board repair & you are a little handy with a soldering iron, Then a self fix is an option, After all you have nothing to lose it seems, Except for the £5 it would cost you for the replacement capacitors. (I did a self fix for my 875's display & all has been perfect since).
Plenty of info & a guide here (Though the pictorial is for a 576 other models have been fixed using the tutorial..
http://www.avsforum.com/t/1246078/how-to-diy-tx-sr576-tx-sr606-etc-hdmi-daughterboard-repair-for-dummies