91 octane should be fine as modern engine management system has the ignition timing controlled by the engine and the knock sensor would retard the ignition timing whenever necessary but then it also depends on the manufacturer's specification as different market might have different octane requirements. The owner's manual should have information on this.

As to the surging sensation and intermittent stalling, that should be due to the idle control system not working efficiently. The idle control air passages tends to accumulate dirt during engine operations and when excessive dirt has accumulated, the idle air passages would be restricted resulting in insufficient air getting past. Cleaning the throttle body and IAC would most probably solve the problem unless you have the Malfunction Indicator Light showing, which means there is an error in the engine management system.

I am having this same exact problem.. my mechanic said it was my air filter hose so I replaced it but it continues to happen.