A PA42 pilot diverted from his filed destination because of severe turbulence and while enroute to SLC experienced several equipment malfunctions including failure of his altitude hold which caused an altitude excursion.

Narrative

I was on a Part 91 flight. I was cleared for the ILS 19 in IMC condition when I encountered severe turbulence and blowing snow. At that moment; I also encountered several failures: autopilot (AP); flight director; yaw damper; partial FMS (PFD went dark). I then decided to execute the missed approach and inquired about the weather in SLC with Salt Lake Center. I decided to head to SLC and requested vectors. ATC cleared me to; 'Direct SLC at 15;000 FT.' While hand flying to in severe turbulence; I was busy recalculating my fuel; troubleshooting the FMS and autopilot problems; picking up weather (ATIS) and getting setup for the approach. Meanwhile I managed to have the autopilot working without yaw damper and I was forced to apply a lot of rudder pressure to maintain proper flight that was adding even more to my already heavy workload. To do so I also had to reset twice the 2 digital (RVSM) altimeters. Weather was VFR over SLC and I had terrain and airport inside 67 miles out. I was still under a lot of work load when ATC cleared me to descent and maintain 12;000 FT with ATIS. I adjusted my altimeter settings and engaged the autopilot in descent mode and armed my altitude capture and hold mode for 12;000 FT. I notified ATC I had airport and terrain in sight. Then ATC called to ask me to maintain 12;000 FT as I was passing through 11;800 FT MSL. My autopilot altitude had just failed and no warning came on. I immediately advanced the throttles to initiate a climb back at 12;000 FT when ATC cleared me for the 'visual' in SLC. I then retarded the throttles and hand flew the approach without any other trouble and landed.