I’ve found that the autopilot won’t really be happy to hold an altitude at speeds lower than 160kts or so. Even worse with the landing gear down as it will get really unstable. It’s happened to me when flying a circuit at an altitude, say for reference 1500FT, the autopilot will barely be able to control the plane at lower speeds in a landing configuration due to it constantly over correcting your pitch, that’s why you’ll get that oscillating action of the pitch.
You should be around 160kts - 180kts around 3000FT, once you get to 2500FT you’ll generally extend the landing gear and thereafter select your landing flaps and decelerate to 140kts or so.
As Chatta290 said, generally your landing speed will be around 140kts in the A320 depending on your weight and headwind component.