At this point it would be the least expensive route to buy a couple of cans of octane booster from any auto parts store or Walmart and dump into your fuel tank.
I hate to say this but your anti-knock sensor may not be working properly.
1 using plus and not premium shoud not do that the computer should adjust to lower octane
2 bad gas. drain tank and fill with fresh or try tooping off as it goes
3 car tuned up...how many miles.. if original change everything
