I just bought a 65 GTO with 398 engine. Seller said he always used 93 octane without ethanol. I put 9 gallons of Shell V power nitro+ in with the half full tank. 5 miles later engine started acting starved then as I reached my driveway it started popping and running rough. Never heard any pinging. Any idea what happened. Obviously it had something to do with the new gas.Thanks for any help you can offer.