Hmm, I run the standard 87 octane gas and haven't run into any problems like that. After replacing the filters and cleaning the throttle body and replacing the sensor, it sounds like it keeps going back to a fuel system problem. Bad fuel injectors maybe, fuel pump?
Did the person you bought it from ever mention any problems before? If not I'd definitely find out.
I would also take it to another mechanic to be sure. They may find something that your other guy just didn't.