THE PITOT STATIC INSTS ICED UP IN AN SMA.

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IN CRUISE FROM BFL TO SJC AT 10000 FT ON IFR FLT PLAN; IN AND OUT OF IMC (MOSTLY IN); TOLD TO DSND TO 8000 FT. RIME ICE WAS BEGINNING TO FORM; SO I ASKED FOR A LOWER ALT. CLRED TO 7000 FT BUT STILL IN IMC AND STILL PICKING UP ICE. ALTHOUGH PITOT HEAT WORKED ON PREFLT; PITOT TUBE SEEMED TO BE ICED OVER SINCE AIRSPD INDICATOR INDICATED CRUISE SPD; THEN '0' KIAS WITHIN A FEW MINS. ALSO; THE ALTIMETER AND VSI WERE VERY ERRATIC IN INDICATING CLB AND DSCNT. WITHIN SECONDS; VSI WOULD READ 2000 FPM CLB; THEN 2000 FPM DSCNT WITH THE ALTIMETER NEEDLE SPINNING BTWN 8000 FT AND 6000 FT. I WAS CLRED TO 7000 FT AND; UPON LEVELING OFF AT WHAT I THOUGHT WAS THE CORRECT ALT; (ALT AND VERT SPD WERE SPORADIC) ATC ADVISED I WAS AT 6300 FT AND TO CLB TO 7000 FT. I INITIATED A CLB. THE ALT AND VERT SPD SEEMED TO CALM DOWN FOR A MIN AND BOTH ATC AND I CONCURRED I WAS AT 7000 FT. I WAS TOLD I WAS OVERTAKING TFC THAT WAS AT 6000 FT WHICH WAS THE REASON FOR MY 7000 FT RESTRICTION. POSSIBLE REASONS FOR MY ALTDEV: 1) ICE ON THE PITOT TUBE BLOCKING BOTH RAM AND STATIC PRESSURE PORTS. 2) RELYING ON INSTS (ALT AND VSI) WHEN ANOTHER INST ON THE SAME SYS WAS OBVIOUSLY INOP AT THE TIME (INCIDENTALLY; AT ABOUT 4000 FT IN VFR; ALL INSTS WORKED PROPERLY). 3) COULD HAVE ASKED ATC TO GIVE ME A LEVEL OFF AFTER INDICATING TO THEM MY INSTS WERE GIVING ME ERRATIC READINGS. FREEZING LEVEL FOR RTE WAS 5000 FT SLOPING TO 11000 FT (N TO S). PIREPS INDICATED ICING AT MY ALT BUT NOT IN MY VICINITY. THEY WERE 30-50 MI AWAY. I COULD AND SHOULD HAVE ASKED FOR DEVS AROUND CLOUDS ONCE I PICKED UP FIRST BIT OF ICE OR POSSIBLY CLBED TO VFR AND CANCELED IFR TO CONTINUE (DEST WAS 3000 SCATTERED). ANYWAY; I THINK THE ICE ON PITOT TUBE CAUSED INST MALFUNCTION WHICH CAUSED MY ALTDEV. MORE RESPECT TOWARDS THE FREEZING LEVEL WILL BE USED WITH/WITHOUT VISIBLE MOISTURE. EVEN THOUGH I WAS FLYING A PLANE THAT IS NOT CERTIFIED TO FLY INTO KNOWN ICING CONDITIONS; I FLEW IT INTO VISIBLE MOISTURE WHILE BEING AT A FORECASTED FREEZING LEVEL. DOES THAT CONSTITUTE A KNOWN ICING CONDITION OR DO I HAVE TO SEE AN ACTUAL RPT OF ICING (PIREP; ETC) ON MY RTE BEFORE I VIOLATE THE 'FLYING INTO ICING CONDITIONS' REG?