Hey Bill,
We set up the same tank this summer. Never had the surging issue. Has the tank been up long? Did you always use the strainer on the overflow? It sounds like Jason posted. Is there anything clogging the air hole on top of the drain pipe? Is there anything clogging anywhere in the drain line? snail or fish? Just want to be sure before you drill the cap larger. I second contacting all glass. Keep us posted.
Scott
