My first thought is to check the elevator throws to ensure they are both the same. Also for warp in the elevator surfaces. I would also check for lateral balance and ensure the plane is in fact balanced laterally. How much throw is in the elevator when this happens, as i have not had this occur in my setup, but my throws are very low in %. For the wing weight, it is not just the weight but where in the wing that weight is, different balsa, different placement of weight concentration, thus lateral balance issues. Also how fast is the plane going when this happens? All above can impact flight characteristics. Just my thoughts, i'm sure others will add more.