KAP140 Autopilot Weirdness

My autopilot has started fighting with me, and I'm afraid it's going to
win.

It's a 3-axis KAP140 in a Skyhawk SP. What it's doing is ... well, it
just starts turning.

After about an hour, it goes into a turn, unannounced. There doesn't
seem to be anything instigating it. It's usually to the left, and
always a "standard" turn.

It ignores the heading bug in HDG mode. It ignores the GPS or VOR head
in NAV mode. It also ignores the ROL mode, and the APR mode - yikes!

I don't know how far it would turn if I left it alone, because the few
times it's happened so far, I've been going somewhere.

Is this a common failure? Does anyone know what might be causing this?
Is my autopilot trying to kill me?