THE FLT ORIGINATED AT ANCHORAGE, AK AT APRX 1400 AST. WHILE FLYING IN THE VICINITY OF KNIK GLACIER, SOME SHEEP WERE OBSERVED & THE PLT CIRCLED IN THE AREA 2 TIMES. DURING THE 2ND TURN, THE ENGINE BEGAN RUNNING ERRATICALLY. REPORTEDLY, THE PLT APPLIED CARBURETOR HEAT & THE ENGINE SMOOTHED OUT. HE THEN DIVERTED TO PALMER WHERE THE ENGINE WAS CHECKED & OPERATED NORMALLY. THEREFORE, HE DEPARTED PALMER AND CONTINUED THE FLIGHT TO WASILLA, AK WHERE HE LANDED FOR A COFFEE BREAK. AT APRX 1550, THEY DEPARTED WASILLA FOR THE RETURN FLT TO ANCHORAGE. THE PLT REPORTED THAT HE CLIMBED TO 3500 FT MSL, & AFTER CROSSING THE KNIK ARM, PROCEEDED ALONG THE MTNS. IN THE VICINITY OF PETER'S CREEK, THE ENGINE BEGAN RUNNING ROUGH. THE PLT REPORTED HE APPLIED CARBURETOR HEAT, BUT THE ENGINE RAN ROUGHER, SO HE DECONTINUED ITS USE. SUBSEQUENTLY, THE ACFT LOST AIRSPEED & THE PLT MADE A CRASH LANDING. THE TEMP & DEW POINT WERE 40 & 23 DEG, RESPECTIVELY. ACCORDING TO ICING PROBABILITY CHARTS. VISIBLE ICING WAS PROBABLE AT CRUISE OR GLIDING POWER.