You can get weird pitch trim failures as a result of the autopilot disconnect switch (the one with the red button) being faulty.
This switch has two sets of normally-closed contacts. One set tells the AP you have pressed it. The other set interrupts the power to the servos, causing their clutches to drop out, so you can hand fly the plane - while you are depressing the red button.
This switch is too flimsy for the current drawn by the servos, these being inductive loads which cause a lot of arcing on the contacts. I took one apart yesterday and there wasn't a lot left of the contacts - after 700hrs of flight.
Doing the AP diagnostics (laptop running hyperterminal, into the AP diagnostic port) reveals nothing because that contact is always OK.
Check your emails/PMs Gary.