Ahhhhh,... here's an explanation for your constantly going into irons. You were sailing upwind trying to beat through both the wind and the current. Suppose that the wind speed is the same as the current speed. As long as you are moving and defeating some of the current speed, you have a slight headwind. When you tack in lightish air you practically come to a stop (relative to the water) and when you stop in this current, you're moving backwards at the same speed as the wind. All of a sudden, you have no wind because the boat is moving backwards as fast as the wind is moving.