D328 CREW HAD ACFT INSTRUMENTATION FAIL IN IMC.

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ALL ITEMS ON CHKLIST WERE CALLED AND MET. TEMP AT THE TIME OF TKOF WAS APPROX 18 DEGS C. CLOSE TO APPROX 10000 FT; WE LOST AIRSPD INDICATIONS ON BOTH SIDES -- CAPT'S AND FO'S. ALTIMETER WAS FLUCTUATING BTWN 5000-11000 FT. WE HAD ONLY ROLL INFO AND NO PITCH INFO. STANDBY ATTITUDE INDICATOR WAS WORKING. HOWEVER; THERE WAS NO ALT AND AIRSPD INDICATIONS ON STANDBY INSTS. WE ADVISED PIT DEP OF THE SIT. WE ASKED FOR VECTORS TO PIT AND DSCNT TO LOWEST POSSIBLE ALT. AT OR ABOUT 3000 FT; ALL INSTS CAME BACK. WE WERE VECTORED FOR RWY 28R APCH AND LANDED VISUALLY. ACFT WAS THOROUGHLY CHKED AND NOTHING WAS FOUND WRONG. ACFT IS BEING CHKED BY COMPANY AND FAIRCHILD TECHNICIANS. CAPT RPTS THAT AFTER INVESTIGATION BY THE ACFT MANUFACTURER; THEY CONCLUDED THE PROBE HEAT WAS NOT ON. THE CAPT REFUTES THIS AS THERE WAS NO EICAS PROBE HEAT WARNING. ALSO; THERE WAS NO BLUE ICE DETECTION WARNING LIGHT ILLUMINATED. THE CAPT STATED THAT THE PROB OCCURRED ONLY WHEN IN MODERATE TO HVY RAIN. THE OUTSIDE AIR TEMP WAS NEVER AT FREEZING OR BELOW. AS SOON AS THE ACFT DSNDED AND FLEW OUT OF THE RAIN THE PROBS STOPPED. IN CONVERSATION WITH THE ACR CHIEF PLT; HE INDICATED THAT THIS WAS A KNOWN PROB WITH THE TURBOPROP VERSION OF THE ACFT. THE CAPT ALSO KNOWS OF OTHER OCCURRENCES WITH THIS MODEL ACFT.