W8 cruise control randomly turning off
 

The cruise control on my W8 has been acting wonky. I turn on the switch on the stalk (gauge cluster light comes on) and when I push the steering wheels set button it may or may not actually set the speed. If it does set the speed, then as I'm driving along the cruise control may or may not randomly switch stop holding speed at any time. Once it stops holding speed, I can usually hit the resume button and it will, but it will inevitably stop again; it may take 5 seconds or 50 minutes though. Sorry to be so vague, but it's really intermittent and annoying. Has anyone seen this behavior before? Any thoughts on what might be up? I'm getting my brakes done tomorrow so if the mechanic has time, I'm going to ask him to take a look too (at $100 an hour). Thanks in advance.

Oh, I don't know if all B5.5 have drive by wire, but I have a throttle stepper motor instead of a cable so I'm guessing it's a fully electric system. No vacuum lines or anything to leak.