I agree with Mac on this one. I would definitely pull the other head to clean those pistons and the combustion chambers. If the other bank of pistons look similar, then you really need to go out and run it up to WOT at least once a week. That helps.

The only other question I have is do you see any blue smoke? Are we dealing with a compelte failure of valve stem seals? Or could it be the gas? There is way too much carbon build-up to be just gas unless you really never get above 1500 rpm while putting around town.

Carbon build-up is a vicious cycle; it builds up and causes pinging which the computer detects so it pulls timing which then leads to more build-up which leads to less timing...

Keep us posted and get some good pictures of the combustion chamber.

Good luck.

John