What was found
STANDBY ISI INSTRUMENT AIRSPEED AND ALTITUDE WAS FLUCTUATING IN FLIGHT AND ON THE GROUND. CREW NOTICED AIRSPEED 280 KTS AT 18,000 FEET AND CONTINUED TO INCREASE AS WE CLIMBED. DIVERTED AN ACCOMPLISHED UNEVENTFUL LANDING. REMOVED FROM SERVICE. REMOVED ISI AND PURGED PITOT STATIC LINES. NO MOISTURE WAS PRESENT. REPLACED ISI AS A PRECAUTIONARY MEASURE. REF Q400 AMM 34-12-16-000-801 AND 34-12-16-400-801. PERFORMED PITOT STATIC LEAK CHECK. ISI CHECKS GOOD. RETURNED TO SERVICE.