THE COMMERCIAL RATED PILOT AND HIS FIVE PASSENGERS DEPARTED COLORADO SPRINGS, COLORADO, ON A PROPOSED CROSS COUNTRY FLIGHT TO ALABAMA. THE AIRCRAFT WAS OBSERVED TO CLIMB TO 150 FEET VERY SLOWLY AND THEN LEVEL OFF. THE PILOT SAID IN A POSTACCIDENT INTERVIEW THAT THE AIRCRAFT WOULD NOT CLIMB OR ACCELERATE NORMALLY. HE SAID AS THE AIRSPEED DETERIORATED, HE WAS FORCED TO MAKE AN EMERGENCY LANDING TO UNSUITABLE TERRAIN. THE INVESTIGATION REVEALED THE AIRCRAFT WAS 311 POUNDS OVER MAXIMUM ALLOWABLE GROSS WEIGHT. DENSITY ALTITUDE WAS 7,500 FEET.