You may have gotten some bad gas, and your dealer is flat out wrong about using any unleaded gas. The manual says to use Premium fuel only, so use it. If you do not have ethanol free fuel in your area, add a splash of Sta-Bil Ethanol Treatment at each fill up. You can get this at almost any auto parts store and most Walmart stores. Add some Sta-Bil to the fuel you have in there now and run it on out (or siphon it out if you think it is bad). Fill up with Premium from now on, not 87 or 89 octane. Personally, I try to only use branded gasoline such as Chevron or Texaco. If I am in a bind, I will get unbranded gas.
Over filling is a possibility. Look inside your gas tank and you will see a little bar. Never fill above that bar as to avoid possible overfilling issues.