What was found
AT APPROX 7:30 PM WE BEGAN A DESCENT OUT OF FL420 FOR OUR ARRIVAL. OUTSIDE AIR TEMP WAS AS LOW AS -46 C AT ALTITUDE FOR 2 HOURS. AT FL340 WE GOT THE YELOW AMBER ENUCIATOR LIGHT JUST ABOVE THE PFD ILLUMINATING "AIU FAIL". OUTSIDE AIR TEMP WAS -31 C. IMMEDIATELY WE LOST THE IAS AND ALT DATA OF THE PFD. THE AUTO PILOT DID NOT DISCONNECT. THIS OUTAGE LASTED APPROX 2 MINUTES AS WE CONTINUED DESCENDING. THE AIU FAIL LIGHT FINALLY EXTINGUISHED. AT 3 HOURS AND 2 MINUTES INTO THE FLIGHT THE PFD FAILED AGAIN FOR APPROX 20 SECONDS. OUTSIDE AIR TEMP WAS +3 C. SAME SCENARIO. AP DID NOT DISCONNECT ALTHOUGH BELIEVE WE LOST THE ALT CAP ARMED MODE AND HAD TO RE-ARM ALT CAPTURE BOTH TIMES. (K)