I STARTED OUT DRIVING ON HIGHWAY ROAD AT APPROXIMATELY 5:45PM TRAFFIC WAS MODERATE, MY SPEED WAS ABOUT 30-35 MPH. ALL OF A SUDDEN I HEARD A CLUNK AND THEN ALARM DING FROM THE DASH AS WELL AS MESSAGE "SERVICE STABILTRAK" THEN "REDUCED ENGINE POWER". I WAS UNABLE TO GO ANY FASTER THAN 25MPH. I DROVE IN THIS MODE FOR ABOUT 2 MILES IN TRAFFIC, WHEN REACHING OPEN HIGHWAY (NO TRAFFIC) THE VEHICLE WOULD NOT SHIFT BY ITSELF. I HAD TO ACCELERATE THEN LET OFF THE GAS, TAP PEDAL THEN THE VEHICLE WOULD SHIFT. I WAS ABLE TO DO THIS MULTIPLE TIMES UNTIL REACHING ABOUT 45MPH. I PULLED OFF INTO A NEIGHBORHOOD, SHUT VEHICLE OFF TO SEE IF THIS WOULD RESET SOMETHING, IT DID NOT. THE VEHICLE WAS IN THIS STATE WHEN I ARRIVED AT THE DEALERSHIP SERVICE DEPARTMENT AT APPROXIMATELY 6:05PM. DEALERSHIP LOGGED FOLLOWING ON SERVICE REQUEST: 5043 FOUND CODE P228C RAN DIAG COULD NOT DUPLICATE PROBLEM FUEL PSI AT SPEC RAISES TO SPEC TEST DROVE ON HIGHWAY DID NOT DUPLICATE CONDITION FOUND NO BULL ON CONDITION TEST DRIVE NUMEROUS TIME, COULD NOT DUPLICATE CONCERN. MILES OUT 5067. *TR