A LEAR 35 HAD A PARTIAL FLT INST FAILURE IN SOLID IMC. THE FO SUSPECTED THAT THE CAPT WAS FLYING POORLY; THEN THE FO REALIZED THAT BOTH ATTITUDE INSTS WERE FAULTY. THE AUX GYROSCOPE SAVED THE DAY AND BOTH MAIN ATTITUDE INSTS LATER CAME BACK ON LINE.

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ON CLBOUT; ATC ADVISED US TO CLB TO 9000 FT AND TURN TO A 360 DEG HDG. THE PF COMPLETED HIS CLB AND TURN AND; AS HE BEGAN TO ROLLOUT ON THE ASSIGNED HEADING; I HEARD HIM SAY 'WHAT THE HECK!' AT THAT TIME I NOTICED THE ACFT BEGAN TO LOSE ALT; I CALLED HIS NAME AND SAID 'ALT' WHEN I NOTICED A LOSS OF 300 FT. THE ACFT THEN BEGAN A RAPID DSCNT OUT OF 9000 FT. WE WERE IMC AT THE TIME. I TOOK CTL OF THE ACFT WHEN THE LOSS OF ALT WAS BEYOND 1000 FT. I INITIATED A RECOVERY; BUT THEN REALIZED MY GYROSCOPE WAS INOP AND WE HAD NOW LOST OVER 2500 FT. I THEN VOICED MY CONCERN THAT I HAD NO ARTIFICIAL HORIZON AND THE ORIGINAL PF TOOK OVER AND INITIATED A RECOVERY WITH THE PEANUT GYROSCOPE. SOON AFTER; HIS GYROSCOPE RETURNED TO OP AND MY GYROSCOPE BEGAN WORKING 15 MINS LATER. THIS INST FAILURE NEARLY KILLED US AND OUR PAX. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE WORK ON THIS ACFT IS NOT YET COMPLETE; BUT THE FO'S GYROSCOPE HAS BEEN REPLACED. THIS MAY HAVE CORRECTED THE PROB AS THE RPTR BELIEVES THAT THERE WAS AN INTERCONNECTED PROB FROM SIDE TO SIDE. NOTHING YET HAS BEEN FOUND WRONG ON THE L SIDE.