TEMPORARY LOSS OF ACFT CTL IN AN ACR WDB AS THE #1 INS FAILS.

Narrative

IN CRUISE WITH THE #1 AUTOPLT ENGAGED; THE #1 INS (WHICH PROVIDES THE INPUTS TO THE #1 AUTOPLT) TUMBLED. THE AUTOPLT; FOLLOWING THE #1 INS COMMANDS; ABRUPTLY ROLLED THE ACFT 30 DEGS TO THE R AND SLIGHTLY NOSE DOWN. MY ADI WAS ROLLING AND I HAD NUMEROUS RED FLAGS. MY FIRST REACTION WAS TO TRY TO OVERPWR THE AUTOPLT MANUALLY. THE FO DISCONNECTED THE AUTOPLT. I ROLLED WINGS LEVEL AND IN THE PROCESS CLBED 400 FT ABOVE ASSIGNED ALTS. I THEN GAVE THE FO ACFT TO FLY BACK TO ASSIGNED ALT. ADVISED ATC OF INS FAILURE AND THAT WE HAD CLBED 400 FT. THEY WEREN'T CONCERNED. WE CONTINUED TO DEST WITHOUT INCIDENT USING #2 AUTOPLT AND FO FLYING AS MY ADI AND HSI WERE INOP. I WOULD LIKE TO MAKE A COUPLE COMMENTS. ALTHOUGH THIS WAS A NON EVENT; ANOTHER SECOND ON THE AUTOPLT AND IN IMC OR DARKNESS AND MOMENTARY PLT DISORIENTATION COULD EASILY RESULT IN AN UNUSUAL ALT WITH SERIOUS CONSEQUENCES. ALL 3 OF US IN THE COCKPIT HAD THE SAME THOUGHT. COULD THIS HAVE CAUSED THE ACCIDENT IN PITTSBURGH? IF THE AUTOPLT WAS ENGAGED; I WOULD BET EVEN ODDS THAT THE AUTOPLT AND NOT THE INS WAS THE CAUSE.