Sometimes older cars need the higher octane - just a function of use and miles put on the engine. Most moderd EFI engines have decent ECM progamming and knock sensors to help adjust for higher octane. I've been using one grade up from regular and noticed much less pinging and better fuel economy to boot.
You could try and blow out some of the carbon build up in the combustion chamber (probably the source of your problem). Sometimes these off the shelf stuff is not strong enough to clear out these deposits - might need something a bit stronger.
Old school method is to pour in or spray in some water or other solvent into the intake - the hot engine will vaporize the water or solvent and help it literally blow out some of these deposits.
Running the car out - couple of WOT applications after a roll - can also help.
Some have had good luck with Auto-Rx or Seafoam - but most of the feedback I see on these forums indicate that it is a temporary fix.