What was found
AT FL360, WE RECEIVED IAS MSG, NR 2 AIRSPEED INDICATION WAS LESS THAN NR 1 & ISIS. DURING DESCENT NR 2 ASI DECREASED AT A RATE EQUAL TO OUR ALTITUDE DECREASE. AT FL310, NR 2 ASI RETURNED TO NORMAL SUDDENLY. DURING AIRSPEED DISCREPANCY WE HAD SPS ADVANCED CAUTION MSG. COMPLIED WITH PITOT STATIC LEAK CHECK IAW WC 34-700-02, LEAK CHECK GOOD. COMPLIED WITH DVI OF PITOT PROBES IAW WC 34-200-02 NO DEFECTS NOTED. INSPECTED PITOT DRAIN LINES IAW AMM 34-13-00 NO DEFECTS NOTED. ISIS FNC GOOD IAW WC 34-700-00.