Took off in my ATR-72, turned on AP, then enabled the FSNav flightplan I had entered earlier. The plane immediately began a turn away from the intended track and began a shallow descent, rather that a climb . I disengaged the AP, manually flew to intersect the correct track, and reengaged the AP. Again both altitude and heading reacted improperly . Reenabled AP, but this time without FSNav input. From here on, HDG hold worked properly, but ALT would see-saw up and down either side of the selected altitude, and NAV was unable to properly track a VOR. Anyone know what caused this, or how to prevent it in future???