The KAP 140 is capable of maintaining altitude to within 50 feet.
However, on the intercept of an altitude it is very common for it to level of +/- 200 feet.
The trick is to press alt when VS appears set the vertical speed for +200fpm or -200fpm and press alt again for it to capture the desired altitude and maintain it.
Or another trick is to start reducing the arte of climb or descent as you are approaching the target altitude.
Reasons:
It controls pitch through the electric trim, it will start levelling off when it reaches the preset altitude but it does not anticipate the level-off.
It therefore usually ends up a little high or low, deviation depending on the rate of climb that was used. A high rate of climb or descend will lead to a larger altitude deviation.
The KAP 140 does not use the same static port as the altimeter, therefore small pressure variations can affect the altitude level-off.
Windows open or vents?
Software inaccuracies?
Cable slack in the flight controls?
Auto-pilot servo's need adjusting?