Someone was telling me about that guy recently. Talk about some bad news.
One of the sim guys told me that the system is designed to take constant input so as to simulate the inherit randomness of moving through an airmass. As such, if it detects no input from the pilot (because you've trimmed it well and are on profile), it will put in it's own input to knock it off the current trim. Of course the result is that it becomes completely unstable and makes you work harder than you would in the plane (not to mention making the first flight back in the plane a relearning experience for the first 10-15 minutes).
The "answer" was to just jiggle the stick an inch or so back in forth while flying straight and level and the system will read that as input and not put in its two cents. I can't remember if that was for the old sims or the new ones, but I found it worked at times, but it wasn't full-proof.