PLT DEVIATED FROM ASSIGNED ALT.

Narrative

ON AN IFR FLT PLAN FROM BUR TO CONCORD; I ENCOUNTERED LIGHT ICING THROUGH 8700' MSL. ICING STOPPED PASSING -8 DEGS C AT 8700' MSL. AT 10000' MSL; 8 MI SE OF GORMAN VOR; ICING STARTED AGAIN AT - 10 DEGS C. PITOT HEAT AND PROP HEAT WAS ON. I REQUESTED LOWER ASAP FROM LA CENTER. THEY SAID EXPECT LOWER 7 MI PAST GORMAN. I ACKNOWLEDGED AND MAINTAINED 10000' TO GORMAN. 2 MI PAST GORMAN THE NOSE PITCHED UP 5-10 DEGS. FULL FORWARD YOKE MAINTAINED 5-10 DEGS NOSE UP. LAST AIRSPD INDICATION WAS 100 AND DECREASING. AIRSPD WENT TO 0; BALL AND TURN NEEDLE WERE ERRATIC DUE TO TURB. VSI VARIED BTWN +500 TO -2000 FPM. ALTIMETER WOULD HOLD; THEN JUMP 500; THEN DROP 500. I ANNOUNCED AN EMER AND THAT I WAS DSNDING. DUE TO VARYING INSTRUMENT INDICATIONS AND LACK OF CTL RESPONSE; I ANNOUNCED I MAY HAVE BEEN IN A SPIN. I SWITCHED ON THE ALTERNATE STATIC SOURCE; STABILIZED ATTITUDE GYRO; BALL AND DG. ALL INDICATION BECAME NORMAL EXCEPT AIRSPD AND I REQUESTED A GND SPD CHK FROM CENTER. THEY SHOWED 140 GNDSPD AND ESTIMATED 110 KIAS. I STABILIZED AND WAS SWITCHED TO BFL APCH. BFL CLRED ME TO 6000'. I CANCELLED THE EMER; CANCELLED IFR AND REQUESTED VFR FLT FOLLOWING AND DSCNT TO 4500'. I COMPLETED CLT CHKS AND REQUESTED A VFR DSCNT TO 2500'. I CONTINUED RADAR FLT FOLLOWING AND CTL CHKS. ALL SYSTEMS WERE NORMAL AND I NOTIFIED COMPANY OF MY PROPOSED ARR TIME AT CONCORD AND COMPLETED THE RUN.