THE PLT REPORTED THAT SHORTLY AFTER TAKEOFF THE ACFT YAWED TO THE RT, LT THEN RT AGAIN. THE PAX IN THE COPLTS SEAT STATED THE RT ENG RPM GAUGE INDICATION NEEDLE WAS FLUCTUATING. THE PLT POSITIONED THE RT ENG MIXTURE CONTROL TO IDLE CUTOFF AND THE PROP CONTROL TO FEATHER. THE PROP CONTINUED TO WINDMILL. UNABLE TO MAINTAIN ALT, THE PLT ATTEMPTED TO LAND ON A HIGHWAY BUT THE ACFT COLLIDED WITH AN UNMARKED POWER LINE, LIGHT POLE, THEN THE GROUND AND WAS DESTROYED BY POSTCRASH FIRE. THE RT ENG WAS RECOVERED AND DUE TO IMPACT DAMAGE, THE PROP AND FUEL SERVO WERE REPLACED. THE ENG WAS PLACED ON A TEST STAND AND WAS STARTED AND FOUND TO OPERATE NORMALLY. THE PROP GOVERNOR AND DAMAGED FUEL SERVO WERE TESTED AND FOUND TO OPERATE NORMALLY. THE DAMAGED PROP WAS INSPECTED AND FOUND TO BE FREE OF PREIMPACT FAILURE OR MALFUNCTION. NO DETERMINATION COULD BE MADE AS TO THE REASON FOR THE REPORTED POWER FLUCTUATION FROM THE RT ENG. A WITNESS STATED GEAR RETRACTION WAS DELAYED AFTER TAKEOFF.