Beech 55 pilot reported descending below the glideslope on an ILS approach when the autopilot apparently malfunctioned.

Narrative

After being vectored to the ILS at ZZZ; I established on the localizer using the STEC 60 autopilot. Once picking up the glideslope; the autopilot started a steep descent ending up well below the glide path. I pulled back on the yoke to lessen the dive; but it continued its downward trend. Before I was able to disconnect it; I had ventured about 900 feet below the glideslope. I disengaged the autopilot and looked up through the windscreen and could visually see the runway approximately 6 to 7 miles ahead. With nothing between me and the runway; I climbed back to the glide path and tried to re-engage the autopilot to try to troubleshoot the problem. It immediately started another descent below the glideslope.At that point ATC called to remind me that I was below the glideslope and the altitude in that area was 1;400 feet MSL. I immediately disengaged the autopilot for the second time and hand flew the aircraft back to the glideslope and made a successful landing. After exiting the active I was informed to contact the Tower Supervisor which was done immediately after exiting the aircraft on the ramp. I informed the Tower Supervisor that I didn't believe that there were any problems with the airport equipment; that my autopilot had malfunctioned; but I was visually in sight of the runway during the event. I will have the autopilot checked by a certified avionics shop prior to using it again.