I haven't had my '84 Specialized Expedition fully loaded for a number of years and don't remember having this problem. I did a test ride with front and rear panniers and about a 45 lb total load and when I hit a cross wind or jiggled the bars around a bit, the front give a shimmy feel. I'm using Blackburn low riders and small to mid size panniers in the front with the heavy items on the bottom. The bike is adjusted fine and wheels are true and 38c tires in good shape. It seems to happen no matter what the speed.
Any recommendations/suggestions? I did a trip in the past with my Bridgesone 400 with a regular front rack (higher mount) and large heavy panniers on it and never encountered a problem. Should I be going heavier in the front?