I don't know whether or not it's related, but in my Cardinal I had a problem with my S-TEC altitude hold when it (the autopilot) was new.  The altitude hold function was divergently unstable - would start to oscillate and then get worse and worse.  Tried all sorts of static systems and finally disconnected the static source entirely and used cabin pressure - seemed to work slightly better.   I was going back and forth with the factory -they initially insisted there was "no problem" and said I should take it back to the installer to have them fix their installation.  Then I broadcast the problem on the net and received replies from all over the world about the same problem.  I sent copies to S-TEC and they immediately said "send it in," so I did.  It came back with no documentation so I called:  "We just checked it out and found no problem."  Oh, yeah?  I installed it and it worked perfectly.  All the earlier S-TEC autopilots, including the 55, used analog circuitry, so the change gain components had to be changed.  They never did admit they had a problem, but I'll bet after that a lot of owners sent theirs back for the same "inspection."

Gary

ES with TruTrack