What was found
PFD AND GPS/NAV1 BOTH REBOOTED AT THE SAME TIME. RESTARTED WITHOUT GOING THROUGH EITHER ITS STANDARD INITIALIZATION OR ITS FAST START ROUTINE. THE AUTOPILOT SEEMED TO BE STILL ENGAGED BUT THE VS SELECTION, QNH, ALTITUDE PRE-SELECT AND HEADING WERE RESET AND THESE RESET VALUES APPEARED TO BE PASSED TO THE AUTOPILOT. THIS OCCURRED AT LOW LEVEL, IN THE CLIMB, IN IMC WITH HEADING AND ALTITUDE ACQUIRE SET ON THE AUTOPILOT. THERE WAS SOME ELECTRICAL ACTIVITY WITHIN 10 MILES. THE AIRCRAFT WAS FLOWN SAFELY BACK TO THE DEPARTURE FIELD USING BACKUP INSTRUMENTS. ENGINEERS SUSPECT EITHER A SPIKE FROM ALTERNATOR 2 OR MFG BELIEVES THIS COULD BE A BONDING ISSUE AND WAS CAUSED BY STATIC. MFG INCIDENT REF: 871500HQ. ENGINEERS SUSPECT THAT THE AHRS DID NOT REBOOT, BUT THIS WAS NOT APPARENT TO THE PILOT.