C182 PLT HAD AN ALT OVERSHOOT IN IMC; IN B90 CLASS E AIRSPACE.

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ATC ISSUED DIRECT HFD AND CLB TO 7000 FT. ACFT ON AUTOPLT. APCHING 7000 FT; SLOWED CLB FROM 500 FPM TO 200 FPM. AT 7000 FT; SWITCHED FROM AUTOPLT COMMANDED CLB TO ALT HOLD. AUTOPLT DID NOT HOLD 7000 FT; BUT CONTINUED TO CLB. AT 7200 FT; ADVISED ATC THAT WE HAD AN AUTOPLT FAILURE AND WOULD BE EXCEEDING 7000 FT FOR A WHILE. DISCONNECTED AUTOPLT AND LEVELED ACFT AT 7400 FT. STABILIZED ACFT BEFORE INITIATING DSCNT TO 7000 FT. ATC NOTIFIED US WE WERE STILL AT 7400 FT. TOLD THEM WE WERE BEGINNING DSCNT. THIS IS SECOND PITCH FAILURE OF AUTOPLT. FIRST PITCH FAILURE WAS PITCH SERVO CAUSING PITCH TRIM TO RUN FULL UP TRIM IN VMC CONDITIONS. THIS FAILURE SEEMS DIFFERENT BUT BECAUSE OF PREVIOUS EXPERIENCE; WE WERE MAINLY TRYING TO STABILIZE THE ACFT BEFORE INITIATING THE DSCNT TO THE ASSIGNED ALT IN IMC. I DID TELL ATC THAT WE WERE HAVING AN AUTOPLT PROB. HOWEVER; I DID NOT SPECIFICALLY ADVISE THEM OF THE FAILURE REQUIRING US TO TURN OFF THE AUTOPLT. BECAUSE THE AUTOPLT CONTINUED TO SOUND AN ALARM; WE PULLED THE AUTOPLT CIRCUIT BREAKER.